Here's one for the techies:
On my recent flight PIT-LAX the ground crew was prepping the plane for takeoff and they pulled over this rather large cart with a hose on it. I looked at it and it had an exhaust pipe on it. They took the hose and hooked it up to the underbody of the plane and then started pumping what I'm guessing air into the plane at full bore for about 10 minutes. about a minute or two before they shut this off they started up the engines and then disconnected the hose. The engine did make what seemed to be a backfire sound when it started up.
Was this some type of "bleed air" or whatever you may call it to get the engine started? If not, what was it and what does it do?