1.

For an example, we have a model shown on the left. Here it is a kite base folded in half and we want to apply an inside reverse-fold.

2.

A typical diagram of inside reverse-fold consists of a S-shaped arrow with a mountain-fold which is shown in (A).

The starting point of the arrow shows an initial point of interested region that we want to track.

The middle point of the arrow shows the actual folding of the model. If you are executing an inside (outside) reverse-fold, then you must apply a mountain-fold (valley-fold).

The final point of the arrow shows where the initial point ends finally.

But I think a valley-fold at the edge replacing the S-shaped arrow is very simple and straight forward to indicate an inside reverse-fold which is shown in (B).

3.

Now, let's apply the inside reverse-fold.

Make sure that outer edges are mountain-folded and the middle line gets valley-folded.

4.

After you completed applying the inside reverse-fold, you will get the one shown on the left.

5.

In a typical diagram, only one mountain-fold and one valley-fold are given (shown in (A)) and you should be able to apply the inside reverse-fold to get to the picture shown in (B).

So if you encounter a diagram with a symbol of one valley-fold and one mountain-fold connected together, then it is the "inside reverse-fold".