The new CentOS 6 OpenVZ kernel has a new memory management model, which supersedes User beancounters. It is called VSwap.
When the Guaranteed Ram limit is reached, memory pages belonging to the container are pushed out to so called virtual swap (vswap). The difference between normal swap and vswap is that with vswap no actual disk I/O usually occurs. Instead, a container is artificially slowed down, to emulate the effect of the real swapping. Actual swap out occurs only if there is a global memory shortage on the system.