Good point Mino... this is a staple move for P-38 drivers. At the right speed, a good 38 jock can pull 3 off no problem.
Here's an excercise to practice controlling your plane in immelmans. Roll out on the runway, and pull directly up into an immelman. Try to see if you can get more than one immelman before you stall. Works best if you get to 200 or so before you lift. Attempt to do immelmans and loops, and here's the tough part, keep them in line with the runway below. Film it so you can see what's going on. This works best in a P-38, but you can do it in any plane. It's great practice for those stall fighting situations. If nothing else, you'll learn just how slow you can make it over the top, and when your plane will snap out on you. It's a great excercise. Credit to my mentor Worr back in the WB days that first showed me this one.