Ah! lovely memories.
A jamming tool, was for us, a length of towel rolled up tightly about six inches long which fit into a pocket sewn into your tights on the rear. If your tights didn't have a pocket you simply stuck it into your tights. This roll was what your partner grabbed to sling you forward, transferring the momentum he gained from diving down from the top of the track.
A jam is what you're doing for a few laps at the bottom of the track, while your partner is at the top of the track recovering from his jam.
At least thats how I remember it.