All ETH and ERC-20 tokens sent to a Split are stored in the contract as balances until a distribution occurs. You can see all the balances in the Earnings section in the top right of a Split's account page. Anyone may initiaite a distribution simply by selecting the balances and hitting the "Distribute" button.
If there is a Distribution Incentive, it will be displayed above the list of balances and whoever performs the distribution will receive that portion of the balance distributed.
There are scenarios in which one may want to distribute a balance and withdraw for all Recipients in the Split. This saves each Recipient from having to withdraw individually, and can be particularly useful if some of the Recipients are unable to withdraw for themselves (e.g., smart contracts unable to execute arbitrary transactions). You can do this by selecting the relevant balances, hitting the dropdown button to the right of "Distribute", and selecting "Distribute & withdraw for all".