How Do They Work?

yes....all TCs have SOME stall

and the mechanic is right....holding the car on a rev will heat up the fluid....

that's why it's better to put the car in neutral or park at idle, unless it's not feasible....in N or P, you turn some of the energy into rotation inside the tranny...the input turns, but not the output
but in gear at a stop, the energy has to be converted to something....that something is heat....the more energy (gas pedal) you put to it without allowing it to move, the more heat it produces