If I recall correctly, it fires relative to the timing of a switch that opens as the cannon swings out from its rest position (the 'home' switch (?).. been a while since I fiddled with this). You can adjust that switch, but it's also the switch that controls the earliest-possible firing as controlled by a human. Changing it only a little bit can modify the rebound angle to catch a post and be pretty saveable or even just safe.

It was changed to fire into the TNT targets in an earlier software revision because it fired up the ramp on a lot of machines, leading to Bowen playing the game for an hour+ in qualifying at PAPA 15... (It's up on youtube sped up with fun comments in captions here:

). There's not really anywhere else 'good' to dump the ball that's neither exploitable nor dangerous given the margin of adjustment error in the wild...