The man penetrates his lady friend in the 'doggy' position. At the point of climax, he puts his hands to the side of his head, palms open and pointing forwards, with the thumbs pressed just above the ears, representing antlers. Moose-esque sound effects/shaking of the head are optional.
With all the noise and shaking, Felicity thought that Roger was having a heart attack. It was only after cleaning up that he revealed that he had been indulging in a spot of moosing. She resolved to position herself in front of a mirror next time he tried to slip her one from behind.