Well, I tried turning on the wipers while they're already wiping from me holding in the wash/wipe button, and (at least in my '10 Focus) the wipers have to be switched to high speed to not get that extra wipe.
Haven't tried it in the Mustang yet ...
'13 Mustang GT SterlingGray, 6-spd man, Recaros, Track Pack. Ordered 6.6.12, born 7.25.12, mine 8.11.12. 2.3L Whipple & a few other things. 591 RWHP, 505 RWTQ