It's a footswitch socket for remote up/down switching of kits
Other details of the Trixer 1 -> 2 upgrade...
Number of factory preset kits increased from 4 to 8
10 user programmable kits
Each of the 10 user kits can use any of the 48 drum sounds (or sound cards)
Tuning for each of the 6 drums in the user kits is programmable
Reverb type programmable for all kits
Separate set of MIDI notes and channels can be programmed for factory kits 1-8 and all user/card kits
Individual channel output mod is separately available (but I've never seen one with this "special" mod)