You have seen that one leg stance in sparring and fighting. It's very common. I had a discussion with some of the student at my school covering some similar concerns about what we practice ans why we don't see it.
With kung fu there are multiple things going on at the same time so in that one leg stance you are seeing multiple techniques that may or may not be done at the same time. In that particular picture there is a leg raise a low block and a what looks like to be a crane technique?
Various Applications
1. The one leg stance can be done to either avoid a sweep, leg check, or to launch a kick. You have seen this technique many times before. As it's usually done without the other hands in position. That one leg stance can also be used to quickly fire off a front snap kick or a front hill kick. I actually showed this in one of my videos when my sparring partner was attacking.
2. The low hand down is a block with the hand directed at knee coming up or if the hand strong enough it could be against a kick before the kick is powerful. This technique of standing on one leg and low blocking is probably best suited for a move that is similar to what I do. I will sweep you so that you lift your leg and as that leg is lifted I'll knee the crap out of you. So this technique would work. If you don't block my knee or interrupt it then I can land that knee into the thigh of the raised leg.
3. The third part looks like a crane technique which we don't have or practice in Jow Ga so I couldn't tell you if what the guy is doing is even correct or not in terms of that hand. In Jow Ga both hands are blocking. There is however a crane technique in Tai Chi that looks similar to what this guy is doing with his form. When you see the hand position like that you can think of it as a defense against a punch. In Tai chi someone would punch at my face with their lead hand I would hook and redirect that punch with my hand shaped like that. In the context of the picture it could be a similar principle where the punch is hook and a heel kick or a snap kick is done shorty after while the punch is being redirected. That's just my best guess but like I said that particular part isn't in my style of fighting where I'll stand on one leg. In my system this technique would be a multiple attacker technique where both hands are blocking, where the leg up is to deal with a possible sweep or knee or just a counter balance while I'm avoiding the sweep.. While the other hand is blocking the other attacker that's on the other side.
The key component that the photo isn't showing is the transition which would help to make more sense of what this guy is doing provided that's a real technique for someone.
In practice standing like this guy does is done for strength building, conditioning, and balance training that will be required to do this in a real fight situation. This is what the application of the form looks like in UFC. Notice the position of the low hand and the knee,