Why?
It’s helpful to think of the movement as a snatch high pull with an added turnover of the bar afterward. This will help reinforce the idea of lifting the elbows high and to the sides before the turnover. The muscle snatch is helpful at lighter weights to learn and reinforce the proper upper body mechanics of the turnover (third pull) of the snatch. At more challenging weights, the muscle snatch will help strengthen the turnover of the snatch. – Catalyst Athletics

How?
Start with the bar on the floor in the snatch starting position. Lift the bar as you would for a snatch until reaching the top of the upward extension of the body. At this point, rather than repositioning your feet and pulling yourself into a squat under the barbell as you would in the snatch, keep the knees straight and the body extended and pull the elbows up as high as possible, keeping the bar in immediate proximity to the body. Once the elbows reach maximal height, turn the arms over to bring the bar the rest of the way up and back into the proper overhead position, punching straight up against the bar and finishing in a tight, aggressive overhead position. The legs must remain straight once extended in the pull. Maintain constant tension against the bar throughout the movement, and make sure the bar is moving continuously—there should be no pausing or hesitation during the lift. – Catalyst Athletics