To initiate the process, but there's more to it than that. In the example of John K he will provide a signed contract with an escrow address to send the BTC to while he is escrowing. Once you have the contract you are responsible for verifying the signatures are valid and his. You are also responsible for checking that the contract came directly from him.

Different escrow providers will use different variations on the process, but one of the most important parts is making sure that you are actually working with a trusted escrow provider and not just some Alt of your trading provider pretending to serve as escrow.