An npn transistor is turned on when the gating is high (voltage at the base node is logic 1) and the switch is positively biased (collector-emitter voltage is positive). It is turned off when the gating is low (logic 0) or the current drops to zero. This is different from the real npn device which is controlled by the base current. Also, contrary to the real device behavior, the npn model in PSIM can block reverse voltage.
Note: The gate node of the npn switch can be connected to a switch gating block or a switch controller ONLY. It can not be connected to any other elements.