Philip said:
1. Knockdown gives you a free trip attack.
2. Improved Trip gives you an extra melee attack, as if you had not used your attack to trip.
Normally, a trip attack is made in place of a normal melee attack.
I take the attack action; I get one attack, which I could use to make a melee attack, a ranged attack, a grapple, a trip, or a disarm. I choose to trip. I succeed; Improved Trip says "Pretend you didn't use that attack to trip, and use it for a melee attack instead".
I make an AoO; I get one attack, which I could use to make a melee attack, a ranged attack, a grapple, a trip, or a disarm (but not a ranged attack). I choose to trip. I succeed; Improved Trip says "Pretend you didn't use that attack to trip, and use it for a melee attack instead".
I successfully hit someone for 10+ points of damage. I get a free trip attack, which I could use to... trip. Naturally, I choose to trip. I succeed; Improved Trip says "Pretend you didn't use that attack to trip, and use it for a melee attack instead". But the attack that I used to trip
can't be used to make a melee attack. It can only be used to trip.
If Improved Trip said "You gain an extra melee attack", it wouldn't be an issue. But it says "You gain an extra melee attack,
as if you had not used your attack to trip"... and that is where the problem comes. If I didn't use that attack to trip,
there's nothing else I can do with it, unlike if I'd tripped with an attack from an attack action, full attack action, charge action, or AoO.
-Hyp.