Leg Kick in breaststroke is something which many people find difficult.
Gone are the days when the legs kick outwards at 45° to the body (like a
frog), also forget the idea of squeezing your thighs together imagining
that it displaces a lot of water backwards.
Both have been shown to be inefficient.
The majority of the propulsion in breaststroke kick comes from the lower
leg and the foot.
So, towards the end of the arm pull start to bring you heels up towards
your backside. Your knees should bend and move forwards (and slightly
outwards), as you do this but they never come right underneath the hips.
There was a time when the top swimmers tried to eliminate movement of
the upper legs completely holding them constantly in a slightly bent
position – try it as a drill to help you to feel the effects of the lower
leg kicking.
When the feet have been raised, the toes are turned outwards and, as the
force of the arm pull dies, the kick is executed by a whipping action of
the lower leg and the foot which brings the toes back together again at the
end in a pointed position.