Definition of twice
Twice means "twice". When you hear the expression "twice" at a poker table, chances are it will be with the expression "run it twice". When you hear "run it twice", it means 2 flops, 2 turns and 2 rivers. The situation occurs during a all in. For example, in a live casino, you are all in preflop with AK. Your opponent has JJ. You know it's about a flip (a 50%-50% situation). To reduce the variance, you ask him if he wants run it twice. If he accepts, 2 full tables will be drawn and you will each play for 50% of the pot size. It is also possible to run it twice from the flop and the turn if all players are all in.
What is the purpose of run it twice?
The purpose of run it twice is simply to reduce the variance. The more hands you play, the less the variance will impact you. Playing run it twice is like playing 2 hands in 1. If you don't pay an extra rake to run it twice, it is strongly suggested that you play run it twice. Plus, it's more exciting than a classic hand.