The starfish humps over the mussel and attaches tube feet to each valve. Each foot exerts only a small force, but the pull is sustained and pooled across hundreds of them, and the starfish can rest individual feet in rotation while the mussel's single adductor muscle cannot rest at all.
Once a narrow gap appears, the starfish everts its cardiac stomach through that opening and begins digesting the mussel inside its own shell. The gap required is small, on the order of a fraction of a millimetre, which is why the animal does not need to force the shell fully open.
A persistent myth holds that the starfish injects a chemical to relax the mussel or that it wrenches shells apart by brute strength. Neither matches the observed mechanics. What is less settled is exactly how tube-foot adhesion is controlled at the molecular level, since the adhesive is secreted and released chemically rather than acting as simple suction.