Backflip: there's a high risk of injury if you don't take proper precautions. Ask two friends for help and try this out on a soft mat in a gym hall.
Stand straight, keep your arms stretched in front of you. Now sit down as if there were a chair behind you. Very important: don't move your knees forward, keep your lower legs at exactly the same position, just move your thighs. Your upper body shifts back- and downwards but remains perpendicular to the ground. At the same time, swing your stretched arms down and behind (see diagramm). Once your posture consists of two 90 deg. angles (hips, knees), do the following actions at the same time:
1. reverse the swing of your arms, throw them from behind in a wide circle far over your head. Do it really fiercly, because it's the only way to gain the necessary rotation. Throw your arms back as far as possible, keep them stretched at all costs, and throw your head back as well (you should see where you land after all).
2. Stretch out your legs as quickly as possible. The main power for jumping off doesn't come from your feet but from your thighs, the sudden burst should travel straight through your legs into your heels.
If you did right the above, everything left is pretty obvious: you land on your hands but the rotational power pulls your legs down (either one after the other or both at the same time). Common follow ups are further backflips or backwards somersaults.
For training purposes get yourself a soft and a hard gym mat. Place them in such a way that you jump off from the hard one but land with your hands on the soft one. Have a friend at each side of yours who grabs you at your back (>support) and under your thighs (>rotation). At your first try let your friends do all the work; fall backwards and let them carry your whole weight. They should put you gently down in handstand position. With every further try jump off a little more until you need no active support from your friends anymore. It's hard work but can be mastered; Today I do it on concrete one after the other, yet I don't dare to finish with a somerault yet:]
Please do this really carefully, I don't want to feel guilty for any accidents of yours. Thanx.
Mam nadzieje ze sobie to przetlumaczysz jakos.