How do you practise knee jumps?

Question

This is a knee jump if you don't know what I mean (just the first demostration)

http://youtube.com/watch?v=YzBoQIw5we4

I want to practise this to improve the height of my jumps.

But what is a safe way to go about it!?? I'm afraid i'll land back onto my knees and hurt myself.....

How is it practised?

Thanks