I don't think you understand what's going on here. Autobalance is never inactive as far as I can tell. You're not fooling it if you switch at the beginning of a round.
The only thing that is happening is this: A lot of people are trying to switch teams at the beginning of the round. So you are more likely to be able to switch from defense to assault simply because there are other people trying to switch from assault to defense. It is much harder to do it in the middle of a round simply because the teams are already set and balanced and very few people are still trying to switch, so you won't be able to unless some people leave the other team or new players come and join yours.
The thing about 'staying dead' at the end of the round: When your equipment screen pops up at the end of the round, you won't spawn in the next round until you've clicked 'done' - just like when you're joining for the first time. People are merely checking what team they've been assigned to before spawning, since trying to change teams once you've spawned will suicide you (and give -1 kill).
Just try it next time: Don't click 'done' right away, but once the round starts and your equip screen is still up hit Tab. You'll see that you have in fact already been assigned to a team - it's just waiting for you to finalize your equipment selection. So as far as autobalance is concerned, it's no different than it would be during the middle of the round. Only, again, that more people are trying to switch, so you are more likely to be able to switch as well.
