The round stingray rests buried in shallow sand with only the eyes and spiracles exposed. Its defensive response is reflexive: pressure on the disc triggers an upward whip of the tail, and the spine enters whatever is above it, usually the top or side of a foot.
The practical countermeasure is a shuffle. Feet slid forward through the sand contact a buried ray from the side, which it detects and flees, rather than pinning it from above. Southern Californian beaches with heavy round stingray populations post this advice routinely, and lifeguard stations at some beaches log hundreds of stings in a summer.
The wound itself is largely mechanical trauma plus a heat-labile venom, which is why immersion in water as hot as can be tolerated is the standard first response. What is often misstated is intent: no stingray attacks a person, and none uses the spine for hunting.