while validating in cakephp, what 'validate'=>'first' does?

[notnull]

I have written in my code like this...\[code\]$this->User->saveAll($this->data, array('validate'=>'first'));$this->User->saveAll($this->data, array('validate'=>false));\[/code\]'validate'=>false will not check for validations in model. But if I replace false with first, then what will it do?Thanks.