Rocket Pool will allow users that are staking, the ability to withdraw a token that backs your deposit, the RPD token. These tokens match the value of ether on a 1:1 basis always when redeeming them at Rocket Pool, provided liquidity of the RPD contract is sufficient. Of course if lots of users have been redeeming these tokens quicker than the RPD contract can refill with ether, then users can sell these tokens on the free market, most likely at a slight discount, to users who are willing to wait for the contract to gain sufficient liquidity again. This means the user always has options to get funds quickly even while staking.
The RPD token contract contains a variable amount of ether in it at any given time. This ether is sourced from Minipools that have finished staking and had users withdraw all or part of their deposit as RPD tokens. This ether is sent to the RPD token contract where it becomes an available source of ether for those wishing to trade in RPD tokens.
When withdrawing RPD tokens, the users incurs a 5% fee which serves a dual purpose. First it prevents abuse of the RPD contract, where a user could keep in permanently drained of ether by staking, then withdrawing RPD tokens and redeeming those immediately. With the 5% fee, the user incurs a small penalty each time.
The second purpose is to help sellers out by incentivising users to buy these tokens on the free market if the user wishes to sell them there, due to the RPD contract not having sufficient liquidity at the time. This means users get an extra 5% ether when trading tokens which would help any seller out who needed funds quickly. In this case, if you had 95 RPD tokens and traded them back into the contract for ether, you would receive 99.75 ether.