It is where the fin is moving side to side not up and down.
In silty conditions this means you don't stir things up anything
like as much so finding your way back out is possible.
It involves twisting your foot a bit and is normally done with
your knees bent so your fins are higher of the bottom.

It feels very awkward at first but is actually a very efficient
way to fin.

nigelH

  AFAIK, there are two main advantages: you don't stir up the bottom,
and you can fin backwards (albeit that requires a bit of practice).
Actually, I don't quite know when normal kick would be better.

  I have seen and used frogkick in two different ways:

1) egpyt style, when you do a move similar to that in breast stroke,
pull the legs up, fins to the side, and then kick back with both legs
using the thigh muscles.  This is a very, very powerful way to swim, and
does well against strong currents.

2) muck style, when you bend kneese up at around 90 degrees, keep them
close together, and then only use the lower part of your leg.  This is
still quite powerful, but you can go over silty bottom without
disturbing it.

  I also find that its easier to turn in place using the 2) style.  Á
la the DIR-hellicopter turns.

  For some footage of frogkicks, try: http://www.frogkick.nl/ =>
Techniken => frogkick

  /Marcin