You drop from a platform raised above the 'ground' onto the 'ground'. You don't jump down as such. You absorb the force on landing and reverse your movement (i.e. jump) as fast as possible. Since you will have forward momentum when you drop down, you will naturally jump up and forwards. There are variations of course, but I'm just describing the basic version - it still is extremely effective though.

Before doing depth jumps you are generally recommended to be able to squat around 2X BW and have good training history with low to medium intensity plyos. Less than that and you risk damage/injury. I suppose in theory if you damage the bone plates you can stunt your growth. So don't jump down from heights you can't handle and/ or doing overly high reps down onto concrete. This is not an exercise where more is better - this is not air alert.

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Height 5'7 Reach approx 7'0
BW @ 23 Jul 08: 145lbs (>13%bf)
Re-training start: Jul 08 (post injury+3 mths rehab)

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