This heat sink style cooler utilizes an aluminum body with fins to extract heat from the fluid in the cooler. Dual pass means the fluid makes two passes through on the way in and back out of the cooler. That allows more opportunity for heat to be removed than a single pass cooler. It also means that the 3/8 inch inlet and outlet hose barbs are on one side allowing it versatility in where it can be mounted, especially with its compact 12 inch length. It provides an alternative to using stock in-radiator transmission coolers or if you don't have room for a larger remote mounted cooler. And it isn't just for automatic transmissions, it could be used for motor oil or power steering fluid as well. Threads for fittings are 1/4-18 NPT