We do "turn off the brain", but not for repetition of well known exercises or repeting technique.
We try to "feel" how to act an react instead of "thinking" what to do. But this is just a different way to perceive what's going on.
Or, in blocked practice of the same movement, you (we, because I do it too) don't have to decide what to do because you know what the attack is, and what technique you're doing, so you don't have to be fully "switched on". One is trying to get better at feeling the nuances of each movement - but - did you ever notice that when you switch techniques in blocked practice (say, after 10 minutes of practicing something you switch to something else) how the first few of the new technique/attack combination, you're a little clumsy until you get into the flow?
In "random" practice, you might be doing the same movement in response to the same attack, but you may not know a-priori which attack is coming at you, so you have to decide a) what's coming, and b) what to do. You may not THINK you're making a conscious decision, but the more experience at random practice we have, the farther back into the attack we "see" and we get better at reading what the attacker is going to do - so - it looks like we're "subconscious", when we're actually more alert, more aware, and more perceptive... (and - I'm using "you" here in the general sense, not any specific person)...