Well, might be known by many rogues and seems like something many of them might have come across while playing, but decided to post here anyway in case someone doesn't know this. Used search and didn't find anything similiar.
Basicly, you want your rogue + random player, prefably a caster (seeing cast bar makes it easier to time your sap). THe idea is to use sap like 0.1 seconds AFTER the spell hitted the humanoid.
What we started with: http://i.servut.us/i/WoWScrnShot_102610_183322.jpg
Right after chain lighting (instant hit after cast, easiest type of spell to get this working): http://i.servut.us/i/WoWScrnShot_102610_183340.jpg
Timing is everything here, practice with instant hit casts like chain lightning or arcane blast. The target is sappable once, then, like you see, it will still target the player who attacked him and is in combat. This could be very useful in PVP when you do it correctly you could get some free dmg in via instant cast and sap right after.
Enjoy