[She may have a good grip, but that's not great leverage when she's sitting in his lap. So she turns her body as they kiss, splaying her knees across him and bracing them against the seat frame to raise her body up, basically climbing up him so she can put her weight to better use.

The chair was definitely not designed for this distribution of weight, however, and as she leans against his chest it pitches backwards abruptly. Her thighs tense and her fists tighten as that makes it very apparent that she's not secured to anything but him, but she's definitely not going to break away because of that, and she presses close as she deepens the kiss from her higher angle.]