as explained here:
http://www.extremenetworks.com/extreme-networks-blog/will-802-11ax-make-80-mhz-and-160-mhz-channels-usable-in-the-enterprise/
Figure 1
Although not often implemented, 802.11ac radios are capable of a dynamic bandwidth operation where an AP can change the channel-width on a per frame basis. In the example shown in Figure 2, an 802.11ac access point that is transmitting on an 80 MHz channel might hear a nearby AP or client station of an overlapping basic service set (OBSS) on channel 56. Instead of deferring transmission on the entire 80 MHz channel, the AP might fall back to transmitting only on its primary 20 MHz channel 52.