The original TRIGR button dates back to Kalypso, and was designed to allow you to attach a GPI or Macro to either the start, the end, or somewhere along the transition.  In this way you might trigger a sound effect to match your wipe, or play back a Clip animation as you dissolved the background.  It was never implemented on Kalypso and I don't think it works on Kayenne either - not a big deal because you can build timelines or macros that do this function.  TRIGR ends up being an extra button to which you can attach a macro - I use it with a macro that lights up specific transition regions (such as BKGD, plus Key 1, plus Key 3, plus Key 6) to mimic the ALL button found on 8000's and 7000's.  If you use a replay move a lot or want another RUN button (fired via macro) and you keep your hand near the Transition Module, you might want to attach the macro that fires the effect to the TRIGR button.