Notso Fantastic the text that
FlareArmed referred to is here:
Take off Training Guide, see Sect 2_42.pdf 2.3.6.5 Manual Braking Techniques. Note that this is a Boeing approved document.
In an RTO stop maneuver, the feet should be up on the rudder pedals and steady, heavy pressure applied until the airplane is completely stopped. Pilots should develop a habit of adjusting their seat and the rudder pedals prior to leaving the gate. The ability to apply maximum brake pedal force as well as full rudder should be checked by both pilots.
If you could post a reference for the Boeing training recommendation then perhaps we could question Boeing about any differences, if any. I would still address the human factors issue that if a crew starts with their heels on the floor then the opportunity for error exists, whereas if the feet are on the rudder bar then opportunity for failure to move the feet into a position to give maximum breaking has been eliminated; which improves safety.
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Airspeed and Upwardness