1. Being away from baby
2. Over active let-down (causing baby to gag)
In this case, expressing a bit of milk before feeding might help to regulate flow
3. Oversupply of milk (expressing might help prevent engorgement)
You can express breastmilk in two ways. Either using God's given natural gift (your hands) or using a breastpump.
The video below shows some helpful tips in expressing breastmilk using hands.