It's when the crank starts 'walking' down the shops lol
Basically, it was only ever a problem on the Evo 1,2,3 and 4, it's when the crank moves around the housing caused by a poorly designed thrust bearing, oil cannot get between the face of the thrust washer and block, and slowly chews away at it, thus making the crank rattle around - you see a lot of people nowadays putting Evo 5 and 6 engines in the 4's to avoid the problem.
Cheers,
Richard