The mech "behind" is the one allocated in the same spot as the targeted one, but in the next line of the squad. If there were no mech in the correspondent spot of that next line, then trample has not effect.
Sample: If your mech (with Trample 20%) successfuly shots for 200 damage a mech in the center slot of the enemy front line, the mech in the centre of the enemy second line (the line behind to the current front one) will take an additional 40 trample damage (even before this mech enters in combat in the front line)
Trample damage can be blocked by Trample shields equipped in the behind mech.
