We first need to understand that there is no direct link between the crankshaft and the transmission input shaft (except in the case of a lock up style converter, but we'll talk about that later). This means that the first function of the converter is to connect the crankshaft and the input shaft so the engine can move the vehicle; this is accomplished through the utilization of a fluidic coupling effect.

The torque converter also replaces the clutch that is required in a manual transmission; this is how an automatic transmission vehicle can come to a stop while still being in gear without stalling the engine.

The torque converter also acts as a torque multiplier, or extra gear ratio, to help the car get moving from a stop. In modern day converters this theoretical ratio is anywhere between 2:1 and 3:1.

Torque converters consist of 4 major components that we need to concern ourselves with for the purpose of explanation.

The first component, which is the driving member, is called the impeller or "pump". It is connected directly to the inside of the converter housing and because the converter is bolted to the flexplate, it is turning anytime that the engine rotates.

The next component, which is the output or driven member, is called the turbine. The transmission's input shaft is splined to it. The turbine is not physically connected to the to the converter housing and can rotate completely independently of it.

The third component is the stator assembly; its function is to redirect the flow of fluid between the impeller and the turbine, which gives the torque multiplication effect from a standstill.

The final component is the lock up clutch. At highway speeds this clutch can be applied and will provide a direct mechanical link between the crankshaft and input shaft, which will result in 100% efficiency between the engine and transmission. The application of this clutch is usually controlled by the vehicle's computer activating a solenoid in the transmission.

Here's how it all works. For the sake of simplicity, I will use the common analogy of two fans which represent the impeller and the turbine. Let's say that we have two fans facing each other and we turn only one of them on- the other fan will soon begin to move.

The first fan, which is powered, can be thought of as the impeller that is connected to the converter housing. The second fan- the "driven" fan can be likened to the turbine, which has the input shaft splined to it. If you were to hold the non-powered fan (the turbine) the powered one (the impeller) would still be able to move- this explains how you can pull to a stop without the engine stalling.

Now imagine a third component placed in between the two, which would serve to alter the airflow and cause the powered fan to be able to drive the non-powered fan with a reduction of speed- but also with an increase of force (torque). This is essentially what the stator does.

At a certain point (usually around 30-40 mph), the same speed can be reached between impeller and the turbine (our two fans). The stator, which is attached to a one way clutch, will now begin to turn in conjunction with the other two components and around 90% efficiency between the crank and the input shaft can be achieved.