When you chose your skater, there will be in the menu "spend money", press on that (make sure you have enough money). Go to "cheats". Buy the cheat that says "make special more special" (it slows the special trick down, but only as long as the trick lasts). Then start playing. (make sure the cheat is on) Find a grindable object and do a special grind (to assign specials go to edit tricks in options). While doing the grind it will be in slow motion and when you finish the grind you'll still be in slow motion. You'll stay in slow motion until you wipe out or bail.