This is one of my favorite yoga poses. It strengthens the back, abs, arms and legs, and even though it's a static pose, it really gets the heart pumping. I like adding this in the middle or beginning of vinyasanas. I'll start in this pose and then slowwwwwwwly lower the back leg down to high lunge. That's great for core strength! (Actually, the exact move I'm talking about is featured in this Slow, Sweaty YouTube video I did.)
The trick here is to make sure the kneecaps are lifted to engage the quads and protect hyperextension of the hamstring. Also, try to keep that lifted leg hip even with the standing leg hip. Reach back and feel your hips are even if you need to. Another trick for this is to point the toes of the lifted leg down to the mat. It looks like mine are a bit off here, but I think it was the angle of the camera...I hope so, anyway. :)
Hope this was helpful to you!