I am wondering this, being a fan of the youtube videos where they do rock slides, and I'm just wondering how they set it up.
Yep. The "j" key freezes physics. Other useful key shortcuts for making rockslides include Shift+C (enter free camera mode, using WASD and the number keypad to move around) and F7. The latter moves objects to the current location of the camera. Simply enter free camera mode, get the camera halfway up a hillside somewhere, and then press F7 to start the show! Alt+the left arrow key engages slow motion - use that in conjunction with the "j" key to make sure you get a good camera angle. Make sure to take plenty of screenshots!