Guys, these bows generally incorporate static limb tips which do not move during the draw and allow for a lower string angle at full draw than otherwise possible. This makes the bow equal to a bow with working tips several inches longer.
Another design element is a forward handle, combine a forward handle with static tips and the recipe is further enhanced.
Stacking is tied to overall bow and specifically limb design more than anything...you can just as easily have a long length bow that stacks if the limb was not designed to move past a certain point.
Superior designs of today allow for these super short bows. The main issue that remains is the ratio of string on the fingers vs. the bows string length: if you have a 50" bow with 3" of string on your fingers VS a 68" bow with 3" of string on your fingers obviously the string contact scenario on the shorter bow will be more likely to cause inputs to the bow upon release.
As for 50" bows, the Sheepeater Spirit was a 50" static recurve that is no longer being made. Timberghost makes some 52" and up bows, Toelke makes a 52" recurve (with working limbs good to 29"), and there are others that like the Thunderchild by Big Jim which is a radical r/d longbow and starts at 52" and up. From personal experience I will tell you the Thunderchild works brilliantly.