What causes the lag

Suppose I am cruising along the highway at 50mph. Then when passing someone, I step on the accelerator all the way to the maximum. When I do this, I notice there is somewhat of a lag. When I step on the gas, there is no effect immediately, then in about a second. The engine suddenly revs up and I can feel the acceleration. What causes this momentary lag and what is going on with the engine and the transmission when I step hard on the accelerator while I am cruising at say 50mph.

The transmission is shifting to a lower gear. The engine doesn’t have enough torque to accelerate when it’s at low RPM’s in overdrive, so the transmission has to downshift before the car can speed up.

Ditto to what Greasyjack said.

"What causes the lag "
Keeping a straight left or bowed wrist at impact helps.