Okay, let's back up a little. I know this might be basic for some, but I'm still going to put this out. The purpose for a clearance between the crank and the main bearings, or the rod bearings and the crank, is because the crank "floats" in the middle of a cushion of oil. This is what prevents metal to metal contact. When the clearance between the crank and the bearings is what they call on the "tight" side, or less clearance, oil pressure as indicated on the gauge will be higher. If the clearance wears down until it is on the "loose" side, or is outside of specifications, then the oil pressure will be lower. When the clearance gets so loose that the oil can't maintain the floating effect of the crank, then oil pressure becomes very low and you get a knock, which is the sound you hear when the crank is making metal to metal contact with the bearings. If you keep driving in this condition, eventually, the bearings will seize up and the connecting rod(s) will break, and that's what is called "throwing a rod". If you're lucky, it will just exit through the oil pan, but usually, you end up with a hole in the side of the block.
Plastigauge is what is used to determine the exact clearance between the crank and the bearings, so that you know ahead of time what that clearance will be. As long as the clearance is within specs, then the crank will safely ride in that cushion of oil.
