In TMP, I thought the intermix chamber was just a conduit that delivered power to the rest of the ship from the nacelles. I also thought the antimatter was up in the nacelles. I was still thinking in TOS and TAS terms, in which the nacelles were more or less implied to house the antimatter reaction.
Then, I got the cutaway poster for the refit and learned that for TMP it was now the other way around and that antimatter was fed into the intermix chamber from the bottom of the secondary hull.
I still think my original interpretation (which isn't contradicted on screen in any way that matters, if at all, even in TWOK) is better.