"云备胎" (yun bei tai) is an upgraded version of "备胎" (bei tai, meaning spare tire). "云" (yun, meaning cloud) is a computer term. A number of spare tires are networked to form a cloud and completely shared. When a charming lady has a need, she can call one or several spare tires. When she doesn't need them, they are still on the cloud. If other charming ladies initiate call requests, these spare tires will take orders from other charming ladies' disposal.