Frogmen often swim underwater using a relaxed body undulation technique that resembles the movement of a seal or dolphin.
Today’s invention is a modified set of swimfins designed to help this kind of stroke.
A pair of regular flippers have regions of velcro attached, as shown.
These can be slapped together, one atop the other, so that the fins stick temporarily together and act like a mermaid’s tail…providing greater potential thrust than the two separate fins.
Surprisingly, I can find nothing of this kind in any of the patent databases.