OS X Yosemite: Secure virtual memory
Secure virtual memory encrypts data being written from random-access memory (RAM) to your hard disk. In OS X, secure virtual memory is always on, reducing possible security risks when data is swapped between your hard disk and RAM.
Your Mac’s RAM contains no information when it’s turned off. Modern computers use virtual memory to eliminate some problems formerly caused by limited memory. Virtual memory swaps data between your hard disk and RAM. If the data written to the hard disk as virtual memory is unencrypted, it can be a possible security risk if the data has not been overwritten and the hard disk is scanned. Secure virtual memory on OS X eliminates this risk.