CALCOP.. I dont think that is what he is talking about.
I completely understand how a heli works, and mine does the same thing. When making adjustments, the heli main rotor loses speed and the heli will fall like a rock until you are done. I almost hit the ground the first time I noticed this... but once I got the adjustment corrected, the heli flew normal.
Mine did it and I was only trying to trim out the tail rotor... (heli was not leaning) It was causing my heli to turn left... So I used my trim adjustment on the control and that is when i had a spike in power and my heli almost fell out of the sky, until the power came back to the main rotors..

Although, he did call it Headspead problem... So I could be wrong.. And if it is when you are trying to move forward, Lean left, lean right, lean back, your heli will drop... Like Calcop said.. unless you give it more power..