The Look Back Rule really doesn't address the speed at which a player must advance or retreat at all. She can leisurely stroll toward a base or hustle in like Pete Rose. Either is fine as long as she is MOVING.
The one stop the runner is allowed between bases is somewhat loosely defined by the rule, in that it says once stopped the runner must "immediately" either advance or retreat. What is "immediate", how long does it last and how do you measure it?
As a matter of standard umpire training, we are taught to interpret the length of "immediately" like this: Just long enough for the runner to pick up the ball, access its location and any pending play, then advance or retreat accordingly.
Generally, once the runner stops, the umpire should begin a silent count of "one-thousand-one, one-thousand-two" and if the runner isn't moving by then she's in jeopardy of being called out.
The Look Back violation in The World Cup game with Japan was an interesting way to end the game. Almost to a man, every umpire I discussed this with thought that the runner was called out too quickly- that is, when the runner stopped after rounding first base, the umpire's "silent count" was closer to about "half-a-thousand" before she banged the runner out.
Also, the umpire that made this call (the plate umpire) didn't seem to be positioned well enough to see both the pitcher and the runner at the same time. That makes it kind of hard to make a good judgment of two simultaneously occuring events.
Not only the timing element of this call may have been violated, but also the very spirit, intent and purpose of the rule itself.
The whole reason that the Look Back Rule was invented in the first place was to put and end to the "cat and mouse" games between runners and the pitcher that were creating long game delays and some farcical baserunning situations. Part of that can be attributed to the relative closeness of the bases as opposed to baseball (60 feet vs. 90 feet). Runners would routinely dance back and forth off the base, advance half-way and just stand there, or perform other stalling tactics to try to force a play by the pitcher, in hopes of either allowing another runner to advance or forcing a bad throw by the pitcher.
You can only image how out of control this might get. A runner stops half-way between first and second and just stands there, while the pitcher holds the ball in one long "standoff. Or, a runner advances, retreats, advances, retreats numerous times while the pitcher pump fakes, stops, pumps again, stops...
To put an end to this, ASA wrote the Look Back Rule (I belive it was around the late 1970's to 1980 period- before my time as an umpire!) The new rule essentially allowed the pitcher to "look back" the runners, forcing an end to cat and mouse games.
The rule isn't there to stop runners from advancing- they still can within certain limits. It also isn't there to be used as a "gotcha!" by the umpires to steal quick and easy outs. The spirit and itent of the rule is to avoid unecessary game delays, nothing more.
