He decides to push her boundaries even further

As Flis obediently does as she's told, James clearly takes this as a sign that she wants to have her boundaries pushed further, and she feels as his other hand, presumably still slippery with sun cream, moves onto her pert bum cheek and a finger snakes it's way towards her puckered little bum hole.

As he presses his finger gently against her arsehole, just applying slight pressure, James continues his manipulation of Flis' pussy and she finds herself mouthing almost silently "Oh god I'm going to cum soon". Her words are meant just for her 'sir', but she must have said them aloud as she hears James respond with; "Not quite yet, you don't!" as he slows the action of his fingers.