From a company that builds these:
Standard heat pipes will transfer heat equally in both directions. If the nominal condenser is hotter than the evaporator, then heat will flow in reverse, from the “condenser” to the “evaporator”. A diode heat pipe is used when it is necessary to prevent heat flow in the reverse direction. There are two basic types of diode heat pipes, Liquid Trap Diodes, and Vapor Trap Diodes. Note that a thermosyphon will also act as a diode heat pipe (the thermosyphon condenser is typically wickless, so liquid is not supplied to the nominal condenser).
A Liquid Trap Diode has a wicked reservoir located at the evaporator end of the diode heat pipe. The wicks in the heat pipe and reservoir are designed so that they can’t communicate with each other. During normal operation, the heat pipe behaves like a standard heat pipe. Heat applied to the evaporator and reservoir causes liquid to evaporate. The vapor travels to the condenser, and capillary action in the heat pipe wick returns the condensate to the evaporator. Since the reservoir wick is not connected to the main wick, the reservoir quickly dries out, and becomes inactive.
When the condenser becomes hotter than the evaporator/reservoir, the role of the evaporator and condenser are switched. Vapor evaporates from the hotter nominal condenser, and travels to the nominal evaporator and the reservoir, where it condenses. Since the reservoir wick doesn’t communicate with the heat pipe wick, any liquid that condenses in the reservoir can’t return to the nominal condenser. In a short time, all of the liquid is trapped in the reservoir. The main part of the pipe contains only vapor, so the only heat transfer from the condenser to the evaporator is by conduction through the heat pipe wall and wick, which has a much, much higher thermal resistance than the resistance during normal operation.
As soon as the evaporator and reservoir become hotter than the condenser, the liquid evaporates from the reservoir, and the heat pipe resumes normal operation.
So they don't contain any mechanical or electronic parts, which qualifies them as passive systems.