When you stake your Hex, you are burning your coins and replacing them with shares in the share pool. Your share percentage is what determines your interest received. When you end a stake, you remove your shares from the share pool and receive Hex back (plus interest) in return.
Share price only goes up.
In order to ensure that longer and larger stakes pay better over time, there is a pricing mechanism built into the contract. Every time a stake is ended the gains for that stake are calculated in the form of a share price which all future stakers will pay to convert their HEX into shares.
You can see how many shares you hold for each stake on the “Stake” tab on go.hex.win