Establishing good grips in BJJ and Judo is so important as very often when you dominate the grip battle and establish your dominant grips you can control and dominate your opponent. I personally work my grips by climbing rope and doing pull ups with my gi. I also rock climb whenever I have the time. Many top BJJ players use rope climbing as a staple of their daily training. Fernando Margarida was said to have spent at least an hour per day on his rope training.
Pull ups are also one of the best ways to build upper body strength.
In a sport like Jiu-Jitsu were you are constantly pulling, grabbing and holding your opponent having grip strength and endurance is very important.
Gi pullups are great for Jiu-Jitsu
Stephan Kesting demonstrates 19 pull-up variations he uses in conditioning for BJJ and submission wrestling.
