As mentioned above the bearing inserts (two halves) are clamped by the main and rod bearing caps. They are kept from moving by the clamping force and locating tabs or dowels. When a bearing spins the lubrication layer fails so the journal rubs against the insert surface. When the insert and main saddle or rod and cap becomes hot enough, their bore expands; relieveing the clamping force; and allowing the inserts to spin inside the bore of the main saddle or rod. Once that happens the insert and bore become the defacto bearing surface but without any lubrication so both begin to heat exponentially. You will know this is happening by a sudden rod knock.
I dismantled a VW 1600 engine that had spun a rod bearing. The rod would not move at all on that journal because it had clamped down on the inserts once it had cooled. The inserts were fused to the crank shaft journal and had to be chiseled off. The rod bore was serverely blued due to the heat and the bore had expanded enough to allow the rod end to bang against the case. The crankshaft probably could have been reground undersize, the rod replaced, and new bearingsinserts installed, but the case was cracked through the main bearing bore