It is my suspicion, although one can never be sure, that this rule was first introduced to stop players sitting on cushion rails or resting torsos on the table to ease the difficulty of reaching the cue ball in those circumstances where it was not easily accessible or within normal reach. A great many players are not fully comfortable with rests and will do all they can to avoid using them. This often sees those players stretching across the table to their fullest extent still knowing, throughout the execution of the stroke, that they must be in contact with the floor. I have seen players get into position to play a stroke because of this, with both legs hanging in the air off the side of the table then forcing one leg down until the very end of the toe of the shoe is barely touching the carpet. Terry Griffiths was one such who would go to this extreme to avoid rest usage.
In those occurrences referees must be extra vigilant to ensure that the rule is not broken, either deliberately or inadvertently.
In considering that a simultaneous strike of the cue ball and the lifting of a toe from the floor must be adjudicated and that there is invariably 7 to 8 feet between the two points of vision, it is difficult in the extreme and maybe even impossible, for anyone to make a definitive decision that the two had occurred in conjunction, just by eyesight.
Keeping in mind that in those difficult circumstances where there are multiple possibilities of the striker fouling, by object balls in close proximity to any part of the body where a cuff foul is a distinct danger, in addition to others close to the cue ball making cueing difficult e.g. and on the most likely event of that foul occurring where a good referee will concentrate attention, it is otherwise permissible to use all the senses, not just eyesight. A watchful eye on the grounded foot and an ear cocked for the sound of the cue on the cue ball can very well be a way around any dilemma introduced by a stretching striker.
In the example below the greater dangers of a foul by the striker are the lifting of the toe from the floor in striking the cue ball and to a lesser extent, a cuff foul on the green ball. The position of the referee slightly back from the pocket closest to the player’s right hip would greatly ease the making of a decision in the event of both or either possibility and still be on the blind side of the player thus avoiding the disturbance of his concentration and perfect positioning may even allow simultaneous direct and peripheral vision to be utilised.
Sec.3 Rule 11 a. iii.