Be really careful doing this. My dad used to do this and one time I decided to try it. I rinsed and rinsed and evidently left it in too long because when I spit it out my gums were white!
OMG - what happened is it killed the entire top layer of my gums! I frantically started brushing my gums with my toothbrush and a thin layer of dead (white) gum tissue came off and underneath was a VERY pink layer of gums!!! Surprisingly it didn't hurt but it was scary and I never did it again after this. Yikes!