'Shell' type bearings like crankshaft mains and big ends are fed with pressurised oil from the oil pump as has been said elsewhere. The rotation of the shaft forms an 'oily wedge' which means that the actual load bearing surface is an oil film, not a metal to metal contact. The egg-head name for this is hydrodynamic lubrication. Roller and ball bearings will run for a bit if they are short of oil but the 'shell' type will fail almost immediately without oil.