You do not provoke an attack of opportunity when you attempt to trip an opponent while you are unarmed. You also gain a +4 bonus on your Strength check to trip your opponent. If you trip an opponent in melee combat, you immediately get a melee attack against that opponent as if you hadn't used your attack for the trip attempt. For example, at 11th level, Tordek gets three attacks at bonuses of +11, +6, and +1. In the current round, he attempts to trip his opponent. His first attempt fails (using up his first attack). His second attempt succeeds, and he immediately makes a melee attack against his opponent with a bonus of +6. Finally, he takes his last attack at a bonus of +1.