Reciting a confessing [Viduiy] during Shofar blowing:
We do not say the confession prayer on Rosh Hashanah. Furthermore, some have the custom to omit the stanza of “Avinu Malkeinu Chatanu Lefanecha” from the Avinu Malkeinu prayer. This is done in order to avoid reciting confessions on Rosh Hashanah in order not to arouse the prosecutor. [Practically, we omit on Rosh Hashanah all stanzas that mention sin such as Chet or Avon and the like.] Similarly, we do not mention Tzidkascha when Rosh Hashanah falls on Shabbos in order not to arouse Divine judgment on a day of judgment.
The Chassidic explanation for why confession is omitted: On Rosh Hashanah we avoid mentioning the confession prayer. This is because the main focus of Rosh Hashanah is the coronation of G-d as our King, as it is on Rosh Hashanah that we accept the yoke of Heaven for the entire coming year. It is for this reason that no confession is made, as confession brings one to a state of remorse and broken heartedness which is a most unwanted atmosphere for the day of the Kings coronation. Rather we must rejoice over the acceptance of G-d to be our King. Now, although certainly one must also repent and have remorse on Rosh Hashanah, as how can there be a coronation if the nation is still in the midst of rebellion, nevertheless this is only a general remorse of one’s lack of discipline in following the Kings commands throughout the past year and a general re-acceptance of the Kings commands. It however is not remorse on each particular sin. A particular confession relates to Yom Kippur which is designated as the day of Teshuvah.
Confessing between Tekios:
In between the sets of the Shofar blowing [of Meyushav] one is to confess his sins quietly. [Some however write that this ruling applies only to the Baal Tokea. Others conclude that practically it is not the Chabad custom to confess between the Tekios. According to all one must stop confessing as soon as the Tekios resume.]
The Chassidic perspective: The confession made between Tekios is not for the sake of fulfilling the Mitzvah of Teshuvah, as otherwise we would have a set order of confession said even not during the Tekios, as is done on Yom Kippur; rather it is an expression of the longing of one’s soul to attach to Hashem. This is the loftiest of moments within Rosh Hashanah in general and the blowing of Shofar in particular. One is able to change his entire essence at this time.
Summary [includes Q&A]:
Although we avoid confessing sins on Rosh Hashanah, and actually omit all mention of sin from the prayer dialect, nonetheless in between the sets of blowing of Tashrat and Tahsat and Tashat and Tarat in the blows sounded before Musaf, it is an auspicious time to confess one’s sins. The confession is to be done in silent, and according to some Poskim is to be said in one’s mind without verbalization. The Chabad custom in this matter is inconclusive, with contradictory directives recorded in various sources. Practically, based on the talks of the Rebbe and Rebbe Rayatz one should at the very least recite a mental confession in his mind during this time, emphasizing a confession for the sake of Dveikus, attaching ones soul to G-d.
Is one to verbalize his confession or is it to be said only in his mind?
 Admur 584:2
 Beis Yosef 584; M”A 584:2 in name of Zohar; Arizal in Shaar Hakavanos 90a; Peri Eitz Chaim Shaar Rosh Hashanah 7; brought in Kaf Hachaim 582:16 and Kaf Hachaim 584:4
 Admur ibid; M”A 584:2; M”B 584:3
 Siddur Admur; Arizal in Shaar Hakavanos 90a; Peri Eitz Chaim Shaar Rosh Hashanah 7; Kaf Hachaim 582:16; Shaareiy Teshuvah 584:2
Ruling of Admur in 584: In 584:2-3 it seems that according to Admur only the stanza of Chatanu Lefanecha is omitted while Selach Umechal is recited. [In 584:2 Admur rules: “One needs to say Selach; Umechal; Mechei; Vihaver Chatoseinu Upeshaeinu as Mechila is greater than Selicha, and Pesha is greater than Chatas, and one must first ask for a minute matter and then add more.” Thus, it seems that even the second opinion in Admur held that sin can be mentioned in the stanzas on Rosh Hashanah. This applies also according to the M”A 584:3. Thus in total there are three customs in this matter: a) Recite everything [Rama] b) Only omit Chatanu. [Beis Yosef] c) Omit every mention of sin. [Arizal] See however Shaareiy Teshuvah 584:2, based on Kneses Hagedola, that those which omit the first stanza but recite the stanzas that contain the word sin are mistaken.
The reason: The Arizal ibid states based on the Zohar that on Rosh Hashanah one is not to mention any sin and hence one is to omit all the stanzas of sin, such as Mechol and Selach Lanu. [Kaf Hachaim ibid]
List of stanzas written by Admur in the Siddur that are to be omitted: “Avinu Malkeinu Chatanu Lefanecha”; “Avinu Malkeinu Selach Umechol Lechol Avonoseinu” “Avinu Malkeinu Michei Vehaver Peshaeinu Mineged Einecha” “Avinu Malkeinu Michok Birachamecha Harabim Kol Shitrei Chovoseinu”; “Avinu Malkeinu Kasveinu Besefer Selicha Umechila”.
 Admur 598:1
 Likkutei Sichos 9 p. 434; Igros Kodesh 22 p. 510
 Siddur Admur; Admur 584:2; M”A 584:2; Shlah 214b; Arizal in Shaar Hakavanos 90a; Peri Eitz Chaim Shaar 26:5; Kaf Hachaim 584:6; 592:17; Minchas Elazar 1:75; Alef Hamagen 590:46; Mentioned in Sichas 1944 p. 31 and 41-42; See Otzer Minhagei Chabad p. 124-125
Wording in Admur 584:2: “Furthermore even actual confession is not to be refrained from being said between the Tekios, as the heavenly prosecutor is in a state of confusion during that time. It is [thus] proper for one to silently confess between the Tekios while the congregation is reciting the prayer of Yehi Ratzon.”
How is there even time to confess? To note however that in the Nussach in the Siddur the Yehi Ratzon is not recited between the sets, as will be explained next. Nevertheless, there is a little bit of time between the sets, as the Baal Tokeia usually takes a small break. [Shlah 214b]
 There are three sets in total, for a total of two confessions recited between the first and second set and the second and third set. [Siddur ibid]
 Arizal ibid; One is not to confess by any other time on R”H.
 The reason: Although in general we refrain from confessing on Rosh Hashanah, nevertheless between the Tekios one may confess as the heavenly prosecutor is in a state of confusion during that time. [Admur ibid; Arizal ibid] The Peri Eitz Chaim ibid brings a contradiction in this matter in whether one may confess on Rosh Hashanah, and he concludes that one may confess silently. Nevertheless, he says that even the quiet confession he would only say in-between the Tekios when the Satan is in confusion. [Kaf Hachaim 584:6; See Piskeiy Teshuvos 592 footnote 18] The Shlah Hakadosh ibid states that at the time of the confession one is to cry and have sincere regret, asking forgiveness from Hashem.
 Otzer Minhagei Chabad 291 in name of Melech Bemisibo; however, from there the matter still remains unclear as the Rebbe concludes that perhaps some are accustomed to do so and so is the implication from the Siddur.
 Rav Raskin p. 527
 Piskeiy Teshuvos 592 footnote 18; See Chapter 2 Halacha 11 p. 202
 Michtav Klali printed in Likkutei Sichos 4; See Igros Kodesh 23:512; Likkutei Dibburim 99-102
 As simply due to that the Satan will not be confused if we confess is not enough of a reason to nullify the Mitzvah of Teshuvah. [ibid]
 Meaning that one confesses his sins for the sake of attachment to G-d, and due to a strong passion to attach to him. This is in contrast to normal confession which comes from a feeling of remorse.
 Sichas of Rebbe Rayatz brought in Otzer Minhagei Chabad 294
 Sichas 1944 p. 31 and 41-42
 Arizal brought in Kaf Hachaim 584:6; Minchas Elazar 1:75 that it is to be verbalized; Simple implication of Admur 584:2 and Siddur which rules it is to be said quietly [implying with verbalization]; Or Letziyon 1:39
The reason it is not an interval: It does not constitute an interval being that it is part of the matters of Davening which are allowed to be said in between the blows. [ibid; 592:7]
 Alef Hamagen 590:46; Derech Hachaim brought in M”B 592:12; Takanas Rav Akiva Eiger; Mahril Diskin 79 [KU”A]; Yabia Omer 1:36; Kaf Hachaim 592:19 regarding Yehi Ratzon, however seemingly regarding the confession he would agree it is to be verbalized, as he concludes there.
 Otzer Minhagei Chabad 292 brings contradictory statements in the name of the Rebbe in two different years of Hamelech Bemisibo. He concludes there based on the Sicha of the Previous Rebbe on the matter of Gaguim that the correct stance is that it should be said only in one’s mind. However, in truth the matter is not clear from the Sicha of the Rebbe Rayatz, as he is merely referring to the feeling behind the confession, and not whether it is to be verbalized or not.