> WHY < does a slow idle stress out the sprag gear so much? What difference does idle speed make?
The slow idle is not directly related to the sprag gear.
The slow idle creates a condition where the engine can be stopped relatively easily if there is a lean-hiccup, cough, or backfire. This is because there is not alot of momentum in the rotation of the engine at a low idle speed.
When any of these things happen, the engine can be stopped, and be pushed into a reverse direction with some force.
Since a sprag clutch is always engaged to the primary gear, and a sprag clutch is a "one way" device that only can work in one direction, the stopping and forceful reversal of the engine can destroy it instantly. It it's not a real bad kickback, then it might survive, but may have some damage that you don't know about, if it continues to work for awhile.
Using a higher idle speed can give some more rotational momentum, so that perhaps a hiccup or cough might not stop the engine, so it won't reverse direction, and this may save the problem from occurring.
So, a slow idle can make the engine stall easier from a variety of problems that might occur at a slow idle speed, particularly if cold. The result of the engine stopping, and reversing direction during the stoppage, with some force, can and will break the sprag clutch system in some cases.