The "Torque Multiplication" is due to the ability of the converter slipping and allowing you to stay at a constant **HIGHER** RPM. The torque comes from the motor the whole time, and isn't really "Multiplied" as it sounds....
Consider the torque converter as a slightly less efficient CVT, Or an infinitely variable ratio differential... Whatever makes more sense to you....
As you floor it off the line, the pressure in the converter builds, and the engine RPMs are able to spin the converter as fast as the pressure will allow up to some point (Stall speed). Your drive ratio also has changed at this point, since things are not 1:1... Your motor is now at 4000rpms (Stall speed) where it would be at 900rpms with a manual transmission. The added torque is because the drive ratio's mechanical advantage allowing you to put more force on the drivetrain. You're appling power 4000 times a second instead of 900.
Once you reach your stall speed, you have very minimal slip allowed in the converter due to the dynamics of it, and it is mostly a power loss to heat from the lack of direct drive. Using a fluid coupling to move a 4000lb car obviously has it's parasitic losses...
Above your stall speed, the converter's purpose has been fulfilled, and is more of a hinderance because of the parasitic loss. If you have the ability to lock the converter, you take that out of the equation, and have a full 1:1 drive ratio through the converter just as a manual transmission with a locked up clutch. You're able to plant more power to the ground rather than waste it in heat. This is shown by higher horsepower on a dyno with a (the same) converter locked as opposed to unlocked.