In addition of being the "defender can't counter" bonus, this 10 TP bonus is also applied if the defender's weapon can break during/after combat (assumes all attacks from defender will hit)
Enemy has 5/20 HP. Player unit has an Iron Sword with 2 uses left and can double attack at 5 damage per hit.
In this scenario, the unit does not get the break bonus. Although the unit can double, only 1 attack is needed to end the battle.
More detail on status staff AI:
Units need to meet a minimum hit requirement in order to be targeted by a status staff. The enemy needs 5+ hit with staff accuracy being calculated from the enemy's start-of-phase position. The initial enemy position is used even if the enemy must move forward to get the target within staff range.
Also, enemies with status staves will retreat a few spaces if a player unit is close, so a status staff can be used at 0-4 hit as long as the 5+ hit test is passed.
Wonder why the FE7 TP mod table has values for class bonus TP even though no classes are defined? It's because the entire TP mod table is copy-n-pasted from FE6 which actually does make use of the class bonuses.
FE6 TP mod table: 0810D728
Pointers to class sets: 085C8674
The most common class bonuses in FE6 are:
Unpromoted Lord: 10 TP
Thief M, Thief F, Bard, Dancer: 15 TP
All magic classes except Priest, Cleric, Troubadour: 5 TP
FE8 also uses the same exact table, and like FE7, doesn't have any classes set to get bonuses.
FE8 TP mod table: 080D8178
Pointers to class sets (all point to 00): 085A8150