It starts when any part of my body crosses the infrared beam. I would say most of the time my head breaks the beam right as I start.
I understand that but there’s still a time lag between the force applied to the ground and the movement that triggers the beam. If you timed a 10m sprint using pressure sensitive starting blocks (like they use in IAAF) this would add the movement time on to your 10m time. The way you do it, reaction time AND movement time are subtracted from the total time.