The Power Snatch is a variation of a full snatch (a.k.a. squat snatch) that limits lower body movement. This exercise is commonly used by weightlifters to help develop power and strengthen a portion of the pull during the snatch. Not only does it build upper back strength, but also an athletes external rotators. It proves to be vital for many athletes to learn this movement as it also helps develop vertical jump.
Much like the snatch, the power snatch can be executed in various positions: floor, below knee, above knee, or hip.
Here is a video of a power snatch from the floor coupled with a snatch from above knee. You will see the difference in catch heights. The power snatch is caught higher and a snatch is caught in the squat position.