Say you invest 0.1BTC when there is a total of 0.9BTC invested, you now own 10% of the bankroll. If a whale comes along and loses 10BTC, you now own 1.1BTC (10% of the total 11BTC) so you will be able to "Move" the funds to your main account and withdraw from there, you will always profit in the long run despite any momentary negative balances because of our house edge.
The results are also provably fair so there is no possibility of us making it seems as if someone is constantly winning.