This listen before transmit feature of 802.15.4 is called CSMA-CA (carrier sense, multiple access-collision avoidance). Wi-Fi speaks at less than 100% duty cycle. 802.15.4 uses the CSMA-CA algorithm to listen for quiet periods and speaks during those quiet periods. With the low power-on latency, short packets, and low data rates, the message is transmitted in a brief cycle well before the WiFi is activated again.