The pocket flex is the basic flex of the equilateral triangle tetraflexagon, but it can be applied to many other flexagons.
One way to do a pocket flex is to fold the flexagon in half and open up a 'pocket' bounded by four leaves.
Depending on the flexagon and the position, you may be able to flex this pocket once, twice or many times.
By itself, it doesn't usually bring the flexagon back to the same position, but is used as a component of more complex flexes such as the
slot flex or pyramid shuffle.
Or you can combine pocket flexes done in multiple places to get back to a similar position.
See the hexaflexagon page for a video demonstrating this flex.
The isosceles dodecaflexagon page has a video with a different example.
The following shows an example of performing a pocket flex on an isosceles triangle dodecaflexagon.