It seems that the din of machzik is not like the din of mitzvos trichos kavana. One can be machzik so long as he has not completely left the mikva, even though he is no longer immersed in the mikvah. This is clear bec. when someone came up but his leg is still in the mikvah, although regarding the water we consider the water on his body attached to be mashlim the mikvah, and perhaps (if you say gud asik) the water in the mikvah to be attached to his head to allow tevila in his hair, we cannot consider him to be immersed in the mikvah (since there are definitely parts of his body that are dry), yet he can still be machzik since he has not completely left the mikva. The rationale must be (as R' Yossi pointed out) that he must accept a level of shemirah before he leaves the mikvah, but does not have to be tovel with kavana as he would have to do a mitzvah with kavana.