It actually doesn't work if you block it too much. The creeps won't aggro you up until they're actually under the tower. The only way to disentangle that after is to draw aggro from behind tower range. This is feeding away a lot of HP. The tanking itself is exposing yourself even if this doesn't happen. The key is actually to block only towards the end when you get vision of the enemy creeps before it's too late.

I'm not completely sure how it works because this has happened to me recently and I used to think like you at one point, but the only logical conclusion is blocking too hard will fuck your equilibrium and tanking won't work.

That being said, this is just my perspective, I could definitely be wrong. I just don't think you really want your creeps right at the edge of your tower anyway because that equilibrium is really easy to disturb, so it doesn't matter if the enemy blocks too hard. You can make up for that with aggroeing and tanking the extra creeps later.