Chapter 3 of John Steinbeck's Of Mice and Men is quite eventful. However, the event which is perhaps more cathartic to the reader is when, during one of his many shows of force, Curley tries to start a fight with Slim. Curley accuses Slim of staring at Curley's wife. Slim, whose personality, charisma, and wit are capable of crushing any argument on site, always beats Curley; but Curley catches a glimpse of Lennie. Lennie, at that point, is daydreaming and smiling at his daydream. Curley thinks that Lennie is laughing at him, so he punches him. Lennie does not fight back and calls for George. George, gives Lennie the command "get him", and Lennie is able to grab Curley's hand as he tried to punch Lennie again, and crushes it.
The embarrasment is too much for Curley but Slim, the intelligent savior, proposed an interesting and clever suggestion:
“I think you got your han’ caught in a machine. If you don’t tell nobody what happened, we ain’t going to. But you jus tell an’ try to get this guy canned and we’ll tell ever’body, an’ then will you get the laugh.”
Basically, the agreement was that Slim would spread the word around to save Curley's reputation as long as Curley does not try anything against Lennie.
The truth is that Lennie's crushing of Curley's hand is no different than having crushed Curley's manhood. The ridiculous way in which he reacts on the floor as he lays in pain makes his situation even more laughable, after all, Curley is a detestable man. Therefore, this agreement was quite needed since it would signify the end of Curley's "reign of terror" among the other field hands.