As light as an eagle:
Control ones sight: One is to be as light as an eagle. This refers to ones sight, that one should close his eyes to avoid viewing evil. Just like the eagle changes his glance very swiftly when he flies across the heavens, so too one should swiftly close ones eyes when seeing evil. Seeing evil is the beginning of sin, as the images which the eyes see is transformed into a desire of the heart which persuades one to sin.
Woman: It is a Biblical prohibition to look at any part of a woman for the purpose of pleasure.
The Rebbe Rashab states the following message regarding the importance of Shemiras Eiynayim [guarding one’s eyes from seeing forbidden sights]:
Forbidden sights and glances is the cause of all matters of evil, as the Sages state that the eyes and heart are the two instigators of sin, as one who looks in the end will come to sin. It is known that the main feeling of pleasure is found in the sense of sight; hence explaining why a blind person does not have pleasure in any matter [and is considered like dead]. It is for this reason that sight arouses pleasure in oneself. This is visibly witnessed even amongst those that are very distanced from evil, that they are able to be aroused towards evil through sight. It captures them into a bucket of filth and leads them to the depths of immorality. Therefore every person who cares for his soul and desires to prevent it from falling into the pits of impurity is to control his sense of sight. Even if this matter is very difficult for him to control, he should arouse within himself a mighty and powerful will to overcome it, having the knowledge of its importance. If one does not control his sight all of his service is considered like naught as he will be unable to affect any true inner feeling of service of G-d, and on the contrary he will descend in a downwards spiral from his level of service. Therefore one must express within himself the strongest will power to overcome it.
Ignore the scoffers: Even if doing so will make a mockery of himself in front of others he should pay them no attention. Is it not that with regards to material matters which relate to ones very life that one ignores the public opinion and acts in accordance to the way he is required to save himself? Certainly then one should do so with regards to spiritual matters which relate to the eternal life of the soul.
The battle is only in the initial moments: The difficulty in controlling ones sight is in truth only on the onset of one’s efforts to take control of his past nature, however with time he will become accustomed to it and its difficulty will cease.
Even a cold glance remains permanent within the soul: Even if one does not feel any conscious feeling or emotion upon glancing at forbidden sights, he should be aware that in truth the sight that he views penetrates to the essence of pleasure that is found in his soul and will eventually, Heaven forefend, erupt itself into his conscious mind. In fact it is for this reason that he is glancing at the forbidden image despite him having no conscious pleasure from doing so, as the essence of his animal soul is so involved within the sight that it is unable at the current moment to reveal the pleasure to the conscious mind.
In summary: “It is an obligation on every individual to overpower himself and control his sight and through doing so he will save his soul from evil and his service of Hashem will be appeasing and effect salvation within his soul”!
The eyes are considered blind for the moment of the forbidden glance:
“In answer to your question regarding the matter of Shemiras Eiynayim, it is understood that there is no place to write this in letters besides for the fact that there is no need to do so being that the matter is explained in the teachings of Chassidus. In general the importance of it can be understood from the saying of the Sages that a blind man is considered like he is dead and hence staring at a forbidden matter which is contrary to the purpose of existence of the eyes is considered a temporary blindness, Heaven save us, and even worse so. It is not necessary to lengthen on this obvious matter.”
To Purchase this Sefer
 Tur; Beis Yosef
Opinion of Admur: The above explanations are taken from the Tur, and are not mentioned by Admur. Possibly the reason for this is because Admur follows the order of the Michaber and Rama, which also did not explain the above. [Ketzos Hashulchan 1 footnote 3]
Opinion of Bartenura: Rabbeinu Ovadia Bartenura comments on this statement that it refers to that one should review his studies and this will cause him to effortlessly be expert in the topic.
 So is implied from commentary of Beis Yosef on the Tur. As for why the term “light” was used, seemingly this is saying that just like the eagle due to his lightness is able to fly quickly past sightings, so too one’s eyes are to be light and swiftly turn away from viewing evil.
 Michaber Even Haezer 21/1; The Riy states it is Biblically prohibited to look at an Erva due to the prohibition of “Lo Sasuru Acharei Levachem Veachareiy Einechem”. However according to the Rambam it is only a Rabbinical prohibition. According to all it is a Rabbinical prohibition to look at a single girl that is not an Erva [i.e. not a Niddah]. According to all it is a Biblical prohibition to entertain forbidden thoughts of any woman, even a single woman that is not an Erva. [Beis Shmuel 21/2] Looking at woman is one of the sins that is most difficult to do Teshuva for being that people think there is no true sin involved as they have not committed any act. In truth it is a grave sin, and is the opening of all matters of immorality. [Rambam Teshuva 4/4] In Kabala it is explained that one who is accustomed to look at woman and does not guard his eyes will be reincarnated into a Reiah. [Shaar Hagilgulim of Arizal, brought in Sefer Chareidim 33]
 Kuntrus Haavoda p. 11: Below is a free translation of selected parts of the Mamar. For the full version of the words of the Rebbe Rashab on this topic refer to the original Kuntrus.
 A proof for this can be found in the Mishneh [Zavim 2/2] that one can see an emission from merely seeing an immoral sight even if he did not think about it. From this we learn the great care one must take to control his sense of sight. [Rebbe in his glosses on Taharas Yom Tov Vol. 9 p. 19; printed in Igros Kodesh]
 Igros Kodesh 19 p. 344 [letter 7342]