If you arrive late to BJJ class and want to get warmed up quickly, then flow rolling is a great technique to try.
Its a useful way to get your body and mind tuned in and ready for BJJ. Building muscle memory, getting your tissue warmed up, and also going at a slower pace will open up and show you new things that you may have missed during a regular rolling session.
As always, listen to your body to ensure you are properly warmed up and choose a training partner who you are comfortable with.