Holding back actual avail coins to “double”

How is that some players are able to hold-back their actual $ of coins so they are able to double a bet -  YET not lose all of their actual bank?  

I’ve seen MANY players double on me with a bank of say $50 and once the game is over I see their bank $ increase immediately to re-fill for another game?

 #1, This is CHEETING #2, It’s unfair to those who bet on their ACTUAL bank and allow the opponent to guarantee they only lose a specific number of coins- avoiding having to pay to replenish.