Machshirei Ochel Nefesh-Must directly be done for the sake of food:[1]

Meleches Ochel Nefesh may only be done if it is not defined as Machshirei Ochel Nefesh, or is defined as Machshirei Ochel Nefesh but could not have been done before Yom Tov.[2] This means that food related Melacha may only be done if one’s body receives direct benefit from the Melacha, such as for example, the Melacha is done with the food that he will eat. If, however, one only receives indirect benefit from the Melacha, such as if the Melacha fixed and prepared a non-food item, and that item will then be used for preparing food, then it is defined as Machshirei Ochel Nefesh and is Biblically forbidden to be done on Yom Tov if it could have physically been done before Yom Tov.[3] This applies even though the intent of the Melacha is to lead to eventual benefit of the body from the food. This applies even if one has no other way of preparing food on Yom Tov, if he does not perform this preparatory Melacha, and it will cause him to lose all his food and end up fasting on Yom Tov. These indirect Melachos are called Machshireiy Ochel Nefesh, or Melachos that prepare an item to prepare food. Nevertheless, the prohibition against performing Melacha of Machshireiy Ochel Nefesh only applies if one was able to perform that Melacha before Yom Tov. However, if one was unable to perform this Melacha before Yom Tov, then it is permitted to perform even this Melacha on Yom Tov.[4] The definition of “unable to perform before Yom Tov” is either if the situation only arose on Yom Tov, such as the food utensil broke on Yom Tov, or if it arose before Yom Tov, but for an important reason completely beyond his control he could not fix it before Yom Tov.[5] (If, however, it could have been fixed some time before Erev Yom Tov, and he delayed it until Erev Yom Tov, and due to uncontrollable reasons he did not have the time to fix it, then it is forbidden to be done on Yom Tov.[6]) [Certainly this prohibition applies if one merely forgot to do the Melacha before Yom Tov, or did not forget, but he was busy and did not have the time to perform it.[7]] See examples brought below for those Melachos forbidden under the definition of Machshirei Ochel Nefesh!

Examples:

Examples of Melachos that are defined as Machshireiy Ochel Nefesh:[8]

1. Chopping wood: One who chops a beam of wood into very small pieces for the sake of making a fire to use for cooking is Biblically forbidden due to Machshirei Ochel Nefesh.[9]
2. Fixing a skewer: Fixing a skewer to use for roasting [i.e. straightening or sharpening] is Biblically forbidden due to Machshirei Ochel Nefesh.[10] This however only applies if one could have fixed the skewer before Yom Tov, or could have made or purchased a new one. If however one could not fix it before Yom Tov, such as if it broke on Yom Tov, or even if it broke before Yom Tov, but for an important reason completely beyond his control he could not fix it before Yom Tov, then he may fix it on Yom Tov. (If, however, it could have been fixed some time before Erev Yom Tov, and he delayed it until Erev Yom Tov, and due to uncontrollable reasons, he did not have the time to fix it, then it is forbidden to be done on Yom Tov.)

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[1] Admur 495/4; Michaber 495/1; Braisa Megillah 7b

The definition of Machshirei Ochel Nefesh: Admur 495/4 “Meleches Ochel Nefesh may only be done if the body actually benefits from the Melacha itself that is done for the sake of Yom Tov. If however one’s body does not benefit on Yom Tov from the actual Melacha, but rather from an item that was fixed and prepared for benefit of man through the Melacha, then it is Biblically forbidden to be done. Although the Melacha is done for the sake of eating food on Yom Tov, nevertheless, since the Melacha is not being done to the actual food, but rather to something that prepares the food, it is therefore considered Machshirei Ochel Nefesh, which is Biblically forbidden to be done on Yom Tov.”; 495/2 “It is only considered Ochel Nefesh if the Melacha is done with the actual food and drink, and thus when one benefits from the food and drink he is benefiting from the actual Melacha.”; Ran 17b; Ramban Shabbos 124b; See Kutrus Achron 495/4 that: This follows the 2^nd explanation in Tosafus Kesubois 7a and Rosh Beitza 19 and so rule the majority of Poskim, which are Rashi, Ramban, Rambam; Rashba, Reiah, Maggid Mishneh; Implication of M”A 518; 1 

Other opinions on definition of Ochel Nefesh: Some Poskim rule the definition of Machshiorei Ochel Nefesh is not indirect benefit but rather any benefit that can be achieved in a way that does not require a Melacha to be done, such as cooking food in a pot rather than fixing the skewer and roasting it. [Admur KU”A ibid that so is implied from 1^st answer of Rosh and Tosafus ibid and so rules Rosh]

[2] Admur ibid; Michaber ibid; Braisa Megillah 7b

[3] See previous footnote for sources ibid who hold of this definition and other opinions who argue

The reason: Although the Melacha is done for the sake of eating food on Yom Tov, nevertheless, since the Melacha is not being done to the actual food, but rather to something that prepares the food, it is therefore considered Machshirei Ochel Nefesh, which is Biblically forbidden to be done on Yom Tov. This is learned from the verse [Shemos 12/16] which states “Hu Levado Yeiaseh Lachem,” which teaches us that only Melachos of Ochel Nefesh may be performed and not Machshirei Ochel Nefesh. [Admur 495/4]

[4] Admur ibid; Michaber 495/1; Beitza 28b

The reason: As the verse [Shemos 12/16] states “Hu Levado Yeiaseh Lachem,” and the Sages expounded this to mean that one may do Melacha for all one’s needs, including preparatory Melachos [i.e. Machshirin] which could not have been performed on Erev Yom Tov. [Admur ibid]

[5] Admur 495/4; M”A 507/7; 509/2; Rashal Beita 4/13; Rashi Beitza 28b; Or Zarua Yom Tov 357; See Admur Kuntrus Achron 495/5

Other opinions: Some Poskim rule that even if one was in a complete state of Oness, such as he did not discover the broken skewer until Yom Tov, he may not fix it, since it was physically possible to be fixed beforehand. [Ran Beitza 15b in name of Tosafus; Shita Mekubetzes Beitza 28b]

[6] Admur ibid; See Admur Kuntrus Achron 495/5; See Nitei Gavriel 2 footnote 12

[7] Implication of Admur ibid and so rule all the following Poskim regarding Ochel Nefesh that could have been done the day before and certainly the same would apply to Machshirei Ochel Nefesh: Admur 495/5; M”A 509/2 and 510/6; Chayeh Adam 80/1; M”B 495/10 [unlike M”B 495/8]; Kaf Hachaim 495/17

Other opinions: Some Poskim rule that if one did not have time to perform the Melacha before Yom Tov then it may be performed on Yom Tov. [M”B 495/8 and Shaar Hatziyon 495/9]

[8] Admur 495/4

[9] The reason: As one’s body does not benefit from the actual wood with which the Melacha was performed, but rather from the cooked food that came as a result of this Melacha. [Admur ibid]

[10] The reason: As one’s body does not benefit from the actual skewer, but rather from the roast that the skewer helped cook. [Admur ibid]