A torque converter contains a propeller that is spun by the engine and another close by which is connected to the rest of the transmission. these two propellers face each other in a chamber filled with oil, so that when the engine-driven prop spins, it pumps oil through the second prop and causes it to spin as well.
Between these two props is a disc with blades in it which catches the oil after it has passed through the second prop, and diverts its flow back around to the engine-driven prop which then picks up that return oil and pumps it into the second prop again.
This disc is cleverly designed so the return oil flow strikes the engine-driven prop blades at such an angle that the energy contained in the return flow adds to the energy being pumped into the flow by the engine-driven prop, in an amount proportional to the difference in rotating speed (called the slippage) between the engine-driven propeller and the second prop.
Recovering the energy in the return flow multiplies the torque force exerted on the second prop by as much as ~3X in the high-slip limit where the second propeller is standing still and the engine is running at speed (which is called the stall torque condition)- as they would be when you put your foot on the gas from a standstill.
In the limit where both props are running at roughly the same speed (the no-slip condition), the torque multiplication factor falls to ~1X.
The action of that extra disc of blades is the key element in getting torque multiplication during the slip condition and is the hardest thing to grasp in the operation of a torque converter!