Reverse mode will reverse everything by pressing X.
Slow Motion mode will slow down everything on a screen by C.
Maybe he just meant flipping the velocities
The reason why undo is limited to five times is because of memory usage, so reverse mode probably isn't possible (unless it is reversing velocities, like @Lord_Bowserinator said), and Slow Motion mode can already be achieved: use tpt.setfpscap(num) in the console, where num is:
- 2 if the FPS should be unlimited.
- The limit for FPS otherwise.
You can also press "F" to view the simulation frame-by-frame.