Trident Mastery 1

If your foe twists free of your trident, they suffer 1d4 points of damage, without your damage modifiers, because it is not an action you are taking.

If you twist your trident as an attack, the target suffers 1d4 points of damage plus your normal damage modifiers, because this is an action you are taking.

In both cases, Damage Resistance applies against the damage.

Q: Does holding someone in your trident count as an attack? Say a character fights with a trident and an off-hand weapon (a shortsword, or a shield to shield-bash with). If on the previous turn they caught an opponent with the trident, on the next turn if they attack with the off-hand weapon, do they suffer penalties for two-weapon fighting?

Yes, you would suffer the penalties for Two Weapon Fighting if you did this.

Trident Mastery 2

When fighting with trident and net, you completely ignore the usual penalties for fighting with two weapons if you have this feat.