A plank on a Swissball is a slightly challenging exercise which adds an element of dynamic stability to the shoulders and through the core.
You should set up by placing your elbows into the Swissball and your legs extended out behind you and your toes gripping the floor. In this position you should aim to have a straight line down from the top of your head down your spine and legs until your heels.
Your tailbone should be tucked under, forcing you to engage through your core. You will find that you continue to make tiny adjustments to your arms positioning on the ball as you work to remain balanced.