Native English speakers, please help me understand this?
"Pam took his hand and led him just aft of the driver's console, guiding him down to a supine position on the deck. She mounted him at once. He tried to sit up, but she didn't let him, instead leaning forward while her hips moved with gentle violence."
Okay so this is an excerpt from a novel. In this particular part there's this guy and the girl who are traveling on a yacht. They kinda get intimate and 'things' are about to happen lol obviously some kinky stuff going on there I know but somehow I fail at visualizing the whole scenario there hehe.. what does it mean that she 'mounted' him at once? Can you explain me how it all happened? Thanks ;)