I'm preparing an encounter with a Shambling Mound. It has the Power Attack feat and it has the Constrict ability. When I run this, if the shambling mound power attacks it would suffer the penalties on its slam attacks, and, if successful, on the grapple checks to start a grapple. However, I'm uncertain about whether the power attack bonus damage would be applied to the constrict damage. Constrict is using 1-1/2 times the strength modifier for damage, so if power attack does increase constrict damage does that mean it would gain +6 damage instead of +4 damage? If the case, the entries would look something like this:
Melee 2 slams +11 (2d6+5 plus grab) or
2 power attack slams +9 (2d6+9 plus grab)
CMB +12 (+16 grapple or
+14 power attack grapple)
Special Attacks constrict (2d6+7) or
power attack constrict (2d6+13)
power attack gives you bonus damage on melee attacks' damage.
constrict isn't normal 'weapon/effect' damage due to an attack roll, but an additional special effect.
power attack wouldn't normally give you free damage on top of a grapple check to initiate (because that doesn't do damage itself),
and it shouldn't with Constrict either just because another effect is kicking in on top of that grapple check.
if you maintained the grapple with the damage option, i would apply power attack bonus there.
constrict damage will also apply there, since it's a succesful grapple check,
but you don't apply power attack TWICE to both normal maintain damage AND constrict...
because constrict isn't damage from the attack itself it doesn't qualify, when maintaining for damage or any other time.
when you make the melee attack to trigger grab, power attack already applies it's bonus damage to the melee attack damage,
not a second time to constrict special effect damage.
so between melee attack+grab and maintain for damage, you should always be getting power attack damage ONCE.
if you choose to not maintain for damage, but for pinning/moving/etc,
you don't get power attack dmg, but normal constrict dmg still applies.