Ok now I got it.
That piece of information means you are getting 1st gear and another gear at the same time.
With two gears engaged at the same time, the output shaft cannot turn, so the engine stalls.
Same principle (two gears at the same time) is used for auto trannys at the drag strip for torque converter lockup of course, that's under control with a tansmission modification.
This was mentioned on page 1, thanks for the help tho.