instead of masturbating, one furiously rubs his dick on a mattress. usually he starts slow and quickens the pace after about 10 mins. Bumphing usually leads to long hard orgasms. The aftermath is usually a wet bed and a underside dickburn. Usually it leaves the person concentrated and satisfied.
Mom: What happened to your bed? Its been wet since this morning!
Son: Sorry mom, I was bumphing before I went to school, it helps my concentration.
the phrase comes from the stereotype that lesbian women like to go camping in the woods. While in the woods they presumably engage in sexual intercourse which leads to the "flooding".
Taylor: What did you do last weekend?
Jackie: I went camping with Jessica and we flooded the woods.