Inside reverse fold
An inside reverse fold is a move where a point is pushed inside itself between its layers, reversing the direction of the crease.
Pre-crease the fold line by folding the point in both directions, then open the layers slightly and push the point inward along the crease until it reverses between them. Flatten the model. It is used for necks, tails, heads and beaks.
The common mistake is skipping the pre-crease, which makes the fold wrinkle. Another is pushing the point too far or not far enough, so check the angle against the diagram before flattening. In the Paper Crane you use it for the neck, tail and head.