Diaz signed a new contract with Zuffa in order to jump over to the UFC. According to Josh Gross' Twitter, Diaz signed a "multi-fight, multi-year" deal which allows him to fight in the UFC and Strikeforce. It includes a provision prohibiting Diaz from boxing.
Sergio Non of USA Today talked to Diaz's manager, Cesar Gracie:
"They were very fair in their dealings with us," Gracie said. "It was kind of a complex contract because of the boxing, so he obviously had to be compensated for not boxing and taking this fight instead."
...
"Nick is going to counter his wrestling with true black-belt jiu-jitsu, Gracie jiu-jitsu," said Gracie, a member of the extended family that formed the grappling style known as Brazilian jiu-jitsu.
The clause allowing Diaz to fight in the UFC and Strikeforce seems redundant, considering, you know, Zuffa owns both promotions. I'm curious if the option allows Diaz to choose from fights presented to him from either organization....
"Nick is going to counter his wrestling with true black-belt jiu-jitsu, Gracie jiu-jitsu," said Gracie, a member of the extended family that formed the grappling style known as Brazilian jiu-jitsu.