The fourth of the Yamas is Brahmachariya. This is usually where people immediately go 'no, not for me' as it is often perceived in the West as restraining from sex. Well, it is and it isn't.
Brahmachariya's literal translation is 'conduct consistent with Brahma' who is the Hindu God of Creation. So that is lovely and vague! We can say that it is to be in harmony with Mother Nature which gives us a bit more of an idea, but a huge amount of scope. Firstly in my mind, Mother Nature intended for there to be sex otherwise human's would be wiped out and there wouldn't be much Yoga left. It means instead to be in authentic relationships which are free from sexual desires and adultery. To many that may seem fair enough, but what you may be noticing in the Eight Limbs of the Yoga Sutras is a very dualistic view; good vs bad. If a human has no sexual desire then would intercourse ever occur? It's just a question I pose to you.
Being in harmony with Mother Nature is a whole lot more that sex. As Brahmachariya is so often associated with that, I thought I'd get that side of things out of the way first of all. It isn't just sexual desire, but any desire at all. It's our desires which lead to greed, to jealousy; therefore to anger, abuse and harm. These emotions are all against the idea of Brahmachariya. In that regard it is a complete surrender to Brahma.
I think a more beautiful and useful way to view it is as 'truth', which is as we now know, one of the other Yamas (they all entwine closely with one another). If the mind is always looking in the direction of, and for truth, then that leads to Brahma; enlightenment can be seen as truth. When this is the single point of focus in a life, then there is no room for desire. I would then question whether truth actually becomes a desire, and so no in keeping with Brahmachariya.
It could also be said that suppressing emotions and desires in this way is actually harmful to the self, and therefore isn't Brahmachariya or Ahimsa (non-violence). It's a very messy web, which is why ancient Yogis had a Guru to guide them and why other strands of Yoga grew in the East. So how do we really apply this to our life?
I think the answer is observation. Instead of suppressing emotions, observe them and question them. 'I desire 'x'. Ok that's totally fine (acceptance, an act of non-violence to yourself). Why do I desire that? What gap is it actually filling (as most desires are filling a void or lack of something usually unrelated)? Is it in keeping with nature and not causing harm?' Or something to that effect. This is actually a more Tantric line of thinking, which is a non-dual perspective. I will write more on this once the Eight Limbs have been covered.