As you can see, there is a "C" on the piece on the left side. The male peg can now slide into that space from the side. Here's another picture for the wings. The red portion of the wings can now be painted separately and inserted into the 2 halves.
Here's a schematic showcasing this. Normally the wings are composed of 3 parts. The red portion is sandwiched in the middle between 2 white pieces. Using the C joint mod, the white part can be assembled first, and the red portion can simply be inserted in. This allows SUPER easy painting without masking.