He lifts his knees, pressing them against her back, if only so he can get his feet under him enough to continue the arch backward until she reaches the grass and he's left above her. It's not the most graceful of repositionings, but it gets him where he needs to go.

Since he's lost the warmth of her anyways, he continues his mouth's path downward. His fingers loosen, but just a touch. It's less that he's thinking of releasing her, and more that he needs to have better range of motion. As such, he keeps twisting her back as his bites move on and around; her side, her hip.

He shoves her knee open, back to the grass. Sweeney's teeth are rougher as he bites the soft skin of her inner thigh.