As Mustafagander has said, this produces a "barn door" type of effect to the free-stream airflow and, as trimotor has correctly stated, is proportional to TAS.
Totally incorrect, I'm afraid. The jet engine is a
reaction engine. The blades in the turbines force the molecules of air in a particular direction (or the reverser cascade, in this case), and as every force has an equal and opposite force the engine (and anything attached to it) is forced in the opposite direction.
What happens to those molecules
after they have left the engine is irrelevant, as those molecules (being in a free gas stream) have no way of transmitting any force back to the aircraft. Your "barn door" is built of gas - not wood!