You leave in low gear and shift it within 600-800 rpm of the launch. You want the low gear launch so you still hit the tire to get out of the beam but you are shifting quick enough that you most likely won't get a full revolution out of the tire. When I do this with the dragster I can go from running 4.80 to 5.20-5.30. All out the car will mph 145-146 and shifting right away to high gear the car will run 141-142mph. I have done this 1/8 and 1/4 mile. No issues with heat and my converter is very loose.
If this is inconsistent for you, the launch rpm may need raised. Where do you normally launch and shift your car at? Is it a glide or 3 speed?