The physical embodiment of a figurative lion-like pose which sometimes occurs when a man ejaculates whilst engaged in sexual intercourse. The lion roar specifically relates to the arched back and head, open-mouthed, eyes closed pose adopted at the moment of ecstasy.
As rapped by Asher Roth in his song Lion Roar;
"Slidin on your side and curves, satisfy the primal urge
Ridin on you back and forth, that's how you make my lion roar"