A Trinary switch will engage the electric cooling fan if it senses the High side pressure reaches approximately 250 psi.
Some feel that the fan needs to come on anytime the AC compressor is engaged. Vintage Air recommends turning on the fan only a when the head pressure starts to get a little too high. Driving at highway speed, there should be enough air flow across the condenser to keep the pressure in the normal range - when the vehicle comes to a stop, the air flow goes away, the head pressure will start climbing and once it reaches approx 250 psi the fan will come on keeping the pressures in the proper range and the AC blowing cold.
Normally the Trinary is wired together with an engine temp sensor so the cooling fan will be triggered on by either the engine getting to the set temp or the head pressure reaches the set pressure.