The force vector of the frog kick is:

[fisherdvm]

I feel that sometime it is upward and backward, and sometime it is backward.... But I am still lousy at it.

 

[John_B]

It depends on whether you are dropping your knees.

 

[jimdiverman]

By its nature, a single finned leg cannot produce a pure backward force, though there is a backward component and a left or right side component. When the legs are moved together in unison, the idea is that the left and right components cancel each other out leaving only the backward force. This is the ideal frog kick for which we all strive.