1- When someone masturbates just after they've finished masturbating, there can be a short period of time between faps.
2- When someone is masturbating for an extremely large ammount of time with no effects (without "cumming") to the point where the arm is extremely tired and the dick feels numb.
3- When someone keeps on masturbating after they just came, generally stopping before a second cum.
In any case, Overfapping is hazardous and could possibly lead to problems with the penis.
Bob: "Yo! Yesterday, I found this amazing video! It was so awesome... I kinda Overfapped..."
Jake: "Really? How many times?"
Bob: "Yo! Yesterday I found this boring ass video... It was sexy though, so I spent the whole 2 hours masturbating, but nothing happened..."
Jake: "Ha, you were Overfapping dude, that's no good..."
Bob: "Yo! Yesterday I was super-horny, so after watching a full length porno movie and cumming I just kept on beatin' my meat!"
Jake: "Seems like you Overfapped bro! ...Why do I even need to know that?"