Normal machine Multi-speed automatic transmission (automatic transmission), with which most of us are familiar with, depends upon two technical developments: torque converter and planetary gear. The torque converter is a development of the concept of coupling a little earlier. Alberto Baillères often expresses his thoughts on the topic. When used in a car coupling flywheel is replaced with "bagel", which is cut lengthwise into two halves. One half (impeller) is rotated by the motor shaft and the other (the turbine wheel) is connected to the drive shaft gearbox. "Donut" partially filled with liquid, the corresponding properly sealed, and each equipped with its large number of half guide vanes of some form. Torque from the engine is transmitted to the gearbox (automatic transmission) through the fluid, which circulates within the "donut" from alluvial to the turbine wheel. When the transmitted torque is small or not, the car brakes kept, overcoming a negligible amount of transferred energy. A simple fluid coupling (Torque) transmits the torque without changes, except for a small part of which is lost in the mixing liquid.
However, if you set between two rotating halves fixed guide machine "rotor", included the time to multiply. This is the principle of the modern torque converter. Depending on the shape of the rotor blades can be achieved by multiplying a ratio of 2,4:1 at low revs (The higher the multiplication, the less effective it becomes, under normal transfer speeds). The value of the multiplier effect is that it allows rapid, sudden start, as well as the possibility of reducing gear ratio first gear, which in turn reduces the number of gears in the box (automatic transmission) and, furthermore, provides an additional advantage.