When you select "Comping" as the record mode, existing takes are kept in the track but are auto-muted by the fresh take. You should be able to see the previous takes by opening lanes.
The comping feature itself is the ease with which these takes can be comped together. Either by speed comping or manual comping. To speed comp, split the clips into manageable sizes to audition them, all takes will split at the same point. Press Shift+space bar to start playback and the use the keyboard arrow keys to navigate the clips.
Manual comping is slightly different but just as quick. Start playback normally and split the clips as the transport is running. Click in the lower half of a clip and the now time jumps back to the start of that clip. Split and click as required.
At any time in either method the split points can be click dragged left/right to move them as well as up/down to adjust the crossfades.