Say you invest 0.1BTC when there is a total of 100BTC invested, you now own 0.1% of the bankroll. If a whale comes along and loses 10BTC, you now own 0.11BTC (0.1% of the total 110BTC) 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.