Constrained profile for Stage3D intends to increase overall reach for the Stage3D APIs to older commonly available GPUs. This allows more players to run your game with GPU rendering (as opposed to Software Rendering, which is a magnitude slower). This is especially useful for browser games.
When you start up Starling with the “auto” profile, it will pick the best available profile for the current hardware automatically. So you don't need to do anything special to support constrained mode. This makes it possible that about 95% of your players will be able to run your game with GPU support via the Flash Plugin.
// Just use the default constructor. starling = new Starling(Game, stage);
Constrained mode has several limitations, though. If you use the standard Starling components, this is not a problem (they all adhere to these limitations); but when you create your own shaders, you might want to keep them compatible with the constrained profile.
The constrained profile has the following limitations:
sln/sge/seq/sneand needs to be replaced with compound
mov/cmpinstructions (this is done under the hood at the cost of using more instruction slots).