This is exactly as the start of the snatch. You want your elbows to make 90 degrees angle at the top, so start with a wide grip. Keep your bum down, chest up and look up, keeping your back straight all the way. Once you get past your knees , explode like you are jumping in the air . Push hard with your legs and pull on the bar, elbows up as high as you can get them.
Always keep your back flat.