To switch places with one's partner in a highly coordinated fashion when showering together in order to optimize the mutual showering experience. This can be done for several reasons: i.e. to minimize water wasted or time spent shivering while outside the stream of water, etc. Generally, it results in the most economically efficient solution to shower sharing. Both partners get water when and only when they need it, and they still get to see each other naked. Win-win.
Jeff: Yo, I heard you showering with your girl this morning. You guys need to hurry it up in there; the water bill is killing me!
Matt: Sorry, bro. She likes shower sex.
Jeff: I get that, but if you're gonna do it at least swaptimize the rest of the time. Shit's getting out of control.