My understanding during training was that if you didn't restart the engine within two minutes things might cool down too much. Right after you restart it in-air you are likely to request more than 10% power. When you are on the ground you have to wait for the oil temperature and coolant temperature to exit the yellow range.Still don't understand the 2 minute start, which really seems to limit single engine training
It doesn't limit single engine training at all. When your instructor "fails" the engine they pull it to idle. When you figure out which engine failed and "feather" it, the instructor moves the lever to 10%, simulating the lack of drag of a feathered engine. You're not allowed to touch that engine lever, but the engine *is* operating.