Hi all :~)
I was trying to figure out how to keep a car type model in my game from getting tilted or knocked out of a level position upon collision with other objects. I don't understand how it works, but it did, and it took me quite awhile to discover this solution, so I'm sharing it. I selected my car (object), added an ALWAYS sensor, an AND controller, and a CONSTRAINT actuator, of the 'orientation' type. I selected "Z" as the axis to be aligned along the reference direction, input a value of ".01" in the Z component of reference direction, and input a value of "0.1" in the MAX field. I tried numerous permutations/variations in these fields before discovering this one that worked. I had read up some on adding constraints in the object menu, which I tried and found did nothing, and later discovered constraints apparently do not work in the BGE, and was informed the only way was through logic bricks. Anyhow, I hope this saves others some time.