about self-timing for 40-yard dash

i used to think timing 40 yard dash starts before my body starts moving, then once i was timing myself at my old high school track, i met a college runner back, and he told me that wasn’t how NFL combines do, they hit start when they see your body starts moving, my question is, to be fair with how NFL players are timed, which way of timing is right? thanks

Usually, most 40 yard dash tests are started when first sign of body movement by the timers… Of course others use electronic devices, but I think the hand held way is mostly used. Self timing would sound like it’s too inaccurate…

thanks, then next time i’ll get somebody to time me