How do exercises help relieve cramps due to menstruation?

Two ways. Exercise can give you natural pain relief. When we exercise, we make endorphins which act like "natural morphine." endorphins make us feel good. Also exercise helps us get rid of prostaglandins, the chemicals which causes uterus cramping. So getting up and getting moving 30-60 minutes every day is good for lots of reasons. Relieving period cramps is one of them. [more info in comments].