Actually, ReefGuy, this is only true if the die-off of organisms on uncured rock produces enough ammonia to create an effective population of ammonia and nitrite converting bacteria (killing most fish in the process), or if live rocks from an existing biologcal filter are placed in the new aquarium. The numbers of beneficial bacteria increase in a curve which follows increasing levels of ammonia and nitrate, and which also lags a few weeks behind that curve. Live rock has no ability to automatically do waste conversions. It will certainly speed the process up, depending on the quality and amount of the rock, and live rock taken from a functioning cycled aquarium will transfer some of the beneficial bacteria to the new tank. Uncured raw live rock will create its own ammonia spike, and cured live rock taken from a tank without a bio-load will not instantly create a biological filter. Biological filters require the continuing production of the quantity of ammonia and nitrate they convert in order to develop, and also to remain effective.
Remove all ammonia producing inhabitants from an aquarium, and the biological filter will quickly experience a dramatic reduction in effectiveness. Replace the ammonia producing inhabitants, and there will be an ammonia spike, though less pronounced and of shorted duration, because many of the bacteria remained, in a dormant state.