there are actually two effects.
effect #1. one is the center to center spacing from one driver to the next. when they are roughly 1/4 wavelength or less, they will more or less sum constructively. with 15" drivers, that is about 1.25 feet center to center. so 1/4 wavelength is 5 feet long, which is 1130 ft/s / 5ft = 226hz. so by crossing up around 450hz, the drivers will *not* be acting as a single source, therefore effect #2 is important.
effect #2. if the drivers are behaving as independent sources, but they are aligned such they are still no different than 1/4 wavelength in total distance from the listener, the sound arriving at the listener will combine constructively and no cancellations will occur. this is the effect we are trying to achieve by moving the outermost woofers forward physically (or pushing the center unit back synthetically by putting a little delay on it*). the difference in the distance that we are talking about in this case is the distance measured from the listening position to the center or each driver. when that distance difference = 1/2 of a wavelength, the sound will completely cancel out at that frequency because the sound from the two sources is arriving exactly 180 degrees out of phase.
so, it is ideal to have effect #1 (which can also be described as drivers being "line arrayed"). almost as good to have effect #2 (which can also be described as having drivers "time aligned").
delay by the way is typically measured in milliseconds, or ms. since the speed of sound is 1130 ft/s under typical conditions, each ms of delay corresponds to roughly 1130 ft/s / 1s = 1.130 feet, or very roughly 1 foot.