What feature on a siamese connection allows one hose to be charged while the other side is connected?

The clapper valve is the correct feature on a siamese connection that permits one hose to be charged with water while the other side remains connected. This design allows for flexibility in operations, as it ensures that water can flow through one hose without affecting the other, which may still be in use or not yet charged.

When connected to a supply, a clapper valve allows water to pass through to one side while preventing backflow from that side to the supply. This is particularly useful in firefighting scenarios where quick deployment of hose lines is crucial. The clapper valve's mechanism essentially acts as a one-way gate, enabling efficient water management during operations.

In contrast, features such as check valves are typically used to prevent backflow in a single line or connection and may not provide the same operational flexibility in a dual-hose setup. Relief valves are designed to release excess pressure, while pressure valves manage the pressure within a system but do not specifically facilitate the operation of a siamese connection.