В IA SDM написано:

Improves the performance of spin-wait loops. When executing a “spin-wait loop,” a Pentium 4 or Intel Xeon processor suffers a severe performance penalty when exiting the loop because it detects a possible memory order violation. The PAUSE instruction provides a hint to the processor that the code sequence is a spin-wait loop. The processor uses this hint to avoid the memory order violation in most situations, which greatly improves processor performance. For this reason, it is recommended that a PAUSE instruction be placed in all spin-wait loops.

Что такое "possible memory order violation"

это относится к мультипроцессорным системам. Подробности - в третьем
томе (и частично в приложении)