It could be a number of things. The two most likely candidates are one, using a special hand kneading technique, and two, using a solid fat (e.g. butter, margarine, shortening) in the crust along with high moisture and double-acting baking powder (in addition to the yeast). I've used a kneading technique for creating croissant-like crusts before, so it might be the one you want to try first. Instead of kneading in a mixer, hand-knead your dough on a flat surface by doing the following:
1) Loosely form a ball of dough
2) Press out your dough into the shape of a rounded rectangle with a thickness roughly 3x your finished thickness and fold in half
3) Pick it up and stretch the dough in the other direction (once again forming a rounded rectangle), place the dough back on your surface, and fold in half
4) Repeat steps 2 & 3 about 16 times or until the dough is smooth
5) Tuck the dough into a ball so that it has a nice tight skin and press gently against your surface to make sure the dough is sealed on the bottom
If you have to punch it down for any reason during the rise, just tuck the ball tighter to force some of the air out.