I never heard of a wing fold....
But what appears to be happening is the following:
How to get from step 18 to 19:
1. Angle bisect the front left and front right flap. (i.e divide the top angle into four equal parts)
2. Squash fold on the creases you just created. (this will give you an upside down kite shape)
3. Focus on the kite shape: Fold the buttom point up so that you connect the two widest points. Fold back down.
4. Use the crease from the previous step and fold the point inside the model. (your kite will now be an Isosceles Triangle)
5. Fold the top left layer to the right.
6. Do a swivel fold. Like what is done here:
here.
7. Fold the right flap back so that you have the triangle back from step 4.
8. Repeat on the right.
Good luck.
Edit: It appears dinogami answered while I was creating my post

. Anyways, dinogami thanks for giving the origin of the term, was kinda curious where it came from.