The ways a trap can lose the water seal that keeps drain air out, and why each one happens.
A trap is the bend under every sanitary appliance that holds a small plug of water, the water seal, which stops foul air and smell from the drain coming back into the room. Level 2 expects you to know the ways that seal can be lost, because a lost seal is a common cause of drain smells.
Self-siphonage happens when an appliance discharges and the moving water pulls the seal out behind it. As a basin empties, the flow can act like a siphon and drag the trap water down the branch, leaving the trap empty. Long or steep branch pipes make it worse.
Induced siphonage is the same effect caused by a different appliance. When water flows down a shared stack or branch, it can reduce the pressure at a connected trap and siphon it, even though that appliance was not used. This is why several appliances on one branch can interfere with each other.
Back-pressure, also called compression, pushes the seal out from the drain side. A surge of water lower down the stack can compress the air below a trap and blow the seal up and out into the appliance, the opposite direction to siphonage.
Slower losses also matter. Evaporation can dry out the seal of an appliance left unused for a long time. Capillary action can wick water out over a rag or debris hanging across the trap. Wavering out, from gusts of pressure in the stack, can splash the seal away little by little, and a leaking trap simply lets the water drain away.
The reason this is on the syllabus is that the cure depends on the cause: siphonage problems are tackled by branch design and ventilation, compression by proper stack design, and the slow losses by use, cleaning or repair. When you meet an empty trap and a smell, work out which mechanism fits before you change anything, and confirm design rules against the current drainage guidance.
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