They then play 'ring toss' with these rings, using their snouts. The rings are 'air-core vortex', invisible, spinning vortices in the water generated from the tip of the dolphin's dorsal fin when it's moving rapidly and turning.
When dolphins break the line, the ends are drawn together into a closed ring. The higher velocity fluid around the core of the vortex is at a lower pressure than the fluid circulating farther away.
Air is injected into the rings via bubbles released from the dolphin's blowhole. The energy of the water vortex is enough to keep the bubbles from rising for a reasonably few seconds of play time.