S-Drill

A pre-dive rehearsal from technical diving in which teammates simulate an out-of-gas emergency and deploy the long hose, verifying gear and muscle memory.

The S-drill, short for safety drill, is a pre-dive rehearsal of the out-of-gas response, standard practice in technical and cave diving and in GUE-style teams. In the water before the descent, one teammate signals out of gas, and the donor responds exactly as in a real emergency: donating the primary regulator from their own mouth on its long hose, typically 5 to 7 ft (1.5 to 2.1 m), switching to the necklaced backup, and fully deploying the hose to confirm it runs free and untrapped. The receiver breathes from the donated regulator, the team confirms, and everything is restowed.

The drill earns its place because it tests the two things that fail in real events: equipment routing and reflexes. A long hose trapped under a canister light cord, a clip left fastened, a backup necklace sitting too low, all are invisible until the hose is actually deployed, and the S-drill surfaces them while the surface is still an option. Repeating the sequence before every dive builds the muscle memory that makes a genuine gas emergency a rehearsed procedure instead of an improvisation. A modified S-drill, deploying just enough hose to verify it is free, is a common abbreviated form on repetitive dives.

Most versions live inside a larger pre-dive sequence, such as GUE's EDGE checks, alongside a flow check of all valves and a bubble check of the whole team at the start of descent. Recreational divers can borrow the idea wholesale: rehearsing octopus donation with your buddy during the buddy check costs 20 seconds and pays in exactly the moment nothing else will.