The main purpose of the tumble turn is to finish one length and begin the next as fast as possible.
In freestyle races, you do not have to touch the wall with your hands.
Any part of your body is fine, and using the feet gets a much faster turn.
The turn is basically a somersault and twist. Here's what you need to do:
Swim towards the wall.
Forward somersault just before the wall.
Place your feet on the wall and stretch your arm out.
Push your feet hard on the wall and twist on to your front.
PRACTICE MAKES PERFECT
There are a number of different ways to practise the turn:
Push away from the side and do a somersault.
Try swimming five strokes and then do a somersault.
Swim towards the wall and do a somersault, putting your feet on the wall as you go over.
Practice pushing away from the side underwater and streamlining - swimming flat and thin with one hand on top of the other. Add a leg kick and start to pull as you reach the surface.
Finally, put the whole thing together - swim, tumble, push off, streamline and swim.
Swim to the wall quickly.
Turn when you see the "T" on the bottom of the pool.
Tuck your head in tight.
Put your feet on the wall approximately 50cm below the surface.
Stretch your arms out before you push away from the wall.