At this stage you are just asking the dog to stay in the sit for a few more seconds. Use your whole hand to reinforce as you move one leg back, click, come in and reward. Gradually build up to moving a step away etc. You are clicking at the point you are furthest away and then will also build up to marking distance and duration. If your dog breaks the sit simple go back a stage. There is no hurry build it up slowly.

Never call your dog out of the stay on a placeboard. ALWAYS go back to your dog.