Searched for this found nothing so here it is.
1.) Que up for arena (Hopefully you get Dalaran Arena)
2.) once in the starting area it will be locked, continue your normal buffing/preparation routine
3.) When the Door Opens Hit the LOGOUT button and your timer will begin going down
Ok, now under the logout session movement will be prevent, and when the water comes to knock you; you wont be moved and remain in the starting location. Enjoy blasting the shit out of your enemies, or healing without problems
FYI, Rember to hit logout on the next water blast!