Vex Ultrasonic Range Sensor (WPILib)

[Ether]

At 6:33 in this video, the instructor mentions that there is WPILib support for decoding the signal from the Vex Ultrasonic Range sensor mentioned at 4:17 in the video.

Can someone familiar with the relevant WPILib code please clarify how the time to receive the reflected ping is determined? Specifically, is a hardware interrupt set up to detect the reflected ping? If so, does the ISR for that interrupt do the timestamping, or does the ISR just set a flag alert the scheduler to run a task which does the timestamping? Or is the reflected signal detected by polling, and if so, what is the polling frequency and priority?