Here you go, from this review: http://www.camcorderinfo.com/content/Sony-HDR-FX7-Camcorder-Review/Other-Features.htm
"Other Features (9.0) Smooth Slow Record – Heavily marketed but of dubious value, the Smooth Slow Record feature can capture up to 6 seconds of video, then stretch it out to play at about 4x slower. The video quality is actually pretty good, but with only six second bursts, this is not an on-the-fly feature. Your shots will have to be set up and timed just right. "
My take is that there is a write time buffer in between bursts that will make it impossible to make this function very usable for wakeboarding.