impact special property:
"This special ability can only be placed on melee weapons that are not light weapons. An impact weapon delivers a potent kinetic jolt when it strikes, dealing damage as if the weapon were one size category larger. In addition, any bull rush combat maneuver the wielder attempts while wielding the weapon gains a bonus equal to the weapon's enhancement bonus; this includes all bull rush attempts, not only those in which a weapon is used, such as Bull Rush Strike, Shield Slam, or Unseat."
from the FAQ about combat maneuvers and weapons:
"Page 199 of the Pathfinder RPG Core Rulebook says, “When you attempt to perform a combat maneuver, make an attack roll and add your CMB in place of your normal attack bonus. Add any bonuses you currently have on attack rolls due to spells, feats, and other effects. These bonuses must be applicable to the weapon or attack used to perform the maneuver.” That last sentence implies that some weapons apply their bonuses on combat maneuver checks, and some do not. So how do you know which weapons do? The answer depends on what kind of combat maneuver you’re attempting, and in some cases what kind of weapon you’re using.
Disarm, sunder, and trip are normally the only kinds of combat maneuvers in which you’re actually using a weapon (natural weapons and unarmed strikes are considered weapons for this purpose) to perform the maneuver, and therefore the weapon’s bonuses (enhancement bonuses, feats such as Weapon Focus, fighter weapon training, and so on) apply to the roll.
For other maneuvers, either you’re not using a weapon at all, or the weapon is incidental to making the maneuver and its bonuses shouldn’t make you better at attempting the maneuver. For example, just because you have a +5 greatsword doesn’t mean it gives you a +5 bonus on dirty trick checks (Pathfinder RPG Advanced Player’s Guide 320), and just because you have a +5 dagger doesn’t mean it gives you a +5 bonus on grapple checks. Of course, the GM is free to rule that in certain circumstances, a creature can apply weapon bonuses for these maneuvers, such as when using a sap in a dirty trick maneuver to hit an opponent in a sensitive spot."
(emphasis mine)
all the above seem to say to me that:
a)sometimes you DO use your weapon to bullrush (impact property says that it also includes bullrush attempts WITHOUT the weapon)
b)when you use a weapon to do a combat maneuver you get the enhancment bonus to the cmb
c)NORMALLY this is for trip/sunder/disarm (normally means that there are occassions where you use your weapons for other maneuvers too)
d)even a direct example of a different combat maneuver than the usual using a weapon
e)GM>all
so, normally you should get your enhancment bonus to your bullrush pilldriver attempt. at least that's how i would play it as a DM
