The pull 'catch phase' is very important in generating power and propulsion.

Many swimmers, especially if they are self taught, will adopt a pull that follows the classic S-shape pull where you pull outwards then in then under the body; or will pull with a straight arm which in fact pushes downwards ...

Here we see Tim practising his arm stroke, which needs more of an extension on the arm entry, glide & catch phase and the right arm needs work to extend to enter the water going forwards and not in front of the head, which it tended to do when Tim got tired. This causes immediate resistance in t...