QQ: When do the effects of Swift Swim take place. I just played a match where I had Mega Swampert, and Mantine on field, switched Mantine for Kyogre to bring the rain, but Swampert didn't go first (others were Groundon, and non-geo Xerneas). So that tells me SS effects take place before turn starts? To be fair I'm totally new to doubles, and VGC. Also Swampert had mega evolved on a previous turn.
Turn order is determined before the turn starts (at move selection, basically). Since Swampert wasn't under Rain before the turn started it didn't go first.







