He payed Georges St-Pierre to come to his hotel and punch him in the belly so he would feel constant pain as if he was hit by a car (which is shown in the trailer).
He's getting stabbed in the film also, getting hurt on the abdomen and hand. Joaquin then started to put a metal clip on his abdomen pinching his skin and he had a bandage for his hand in which he put small nails underneath so he could press and also feel the pain (Sometimes even making himself bleed by accident).
My friend asked him why all the trouble and Joaquin's response was simple : He doesn't want to think about those parts of acting, by really being hurt, it helps him concentrate on other thing and not forget where he's supposed to feel pain.
And also, be prepared for gigantic testicles (during the bathroom scene If I remember correctly)