What is the most effective way to improve the efficiency of swimming strokes?

By applying "hydrodynamic".

Hydrodynamics is a branch of material science that deals with the movement of liquids and the powers following up on solid bodies submerged in liquids and in movement in respect to them. Liquid hydrodynamics assumes a part in a swimmer's general execution, in light of the fact that the powers delivered in and by water can either increment or decrease a swimmer's speed.