I researched this and for my BFP cycle, it was after. I believe I read to wait at least 5 minutes after but no more then 45 minutes. What I recall was it grabs the swimmers and moves them closer to the target destination.
I realize I didn't include this in my history so thank you for reminding me.
I cant do any searches to confirm where I found this right now since Im at work and it is liable to setoff the internet police