The only thing I can think he might be referring to is the aquabee pumps, like my MCE600 has, have a little flap that flips one way or another depending on which direction the impeller decides to spin. That way it always pumps well. But if that gets jammed it won't flip so the pump won't pump as much.
But...this is far from a secret, it says pretty much that in the manual... The manual fell open once and I saw it...I didn't RTFM, honest, that would be unmanly! :lol: