Rotoscoping is the art of painting directly on frames . Before Vector Paint and Paint Effects, roto was done in AE by animating masks which is still a valid way to work. Some people may find the act of painting a stroke more intuitive though.
When rotoscoping with AE remember that the PgDn button will advance one frame, and the PgUp button will go a back a frame.
You can also advance a custom amount by changing the custom duration in the duration pop-up menu and pressing Cmd+PgDn or Cmd+PgUp (Ctrl+PgDn or Ctrl+PgUp).
The following are some additional tips to keep in mind.
To zoom in the layer window, press the Period key.
To zoom out the layer window, press the Comma key.
Regardless of what tool you're using, holding the space bar will change your tool to the hand tool for fast and easy navigation in the Layer or Composition Windows.