A torque converter is the device that transmits power from engine to the transmission. It is a hydraulic fluid coupling. It uses vanes to transmit torque via transmission fluid from input to output. There is always a slight difference in input to output speed resulting in about 10% power loss. Manual transmissions dont suffer this loss which is why they get better milage.
You get a TC with a higher stall speed so that you are taking off in your power band.
Say your truck makes the most torque at 3500 rpms, and it is a dog below that. So you get a TC that has a stall speed of 3500rpms. Meaning it won't engage untill than. Almost like holding in the clutch on a manuel tranny and not letting it out till you are at 3500rpms. It will be a pain in the *ss to drive on the street, but will be a monster off the line.
For instance, my dad had a 46 ford with a 502 big block. He had a 2700 rpm torque converter on it, so at a redlight you couldn't take off without at least barking the tires. And you couldn't just idle through a parking lot, you had to have the revs at least 2700 or it would be kicking in and out. That damn thing got me a noise violation ticket for barking the tires at a redlight with a cop behind me.