Updated: Aug 24, 2019
Check out this microburst caught on camera in the Raleigh, NC area. The important thing to notice is just how benign this looks underneath the cloud deck...lots of locations with blue skies in the distance.
Below is the dual-node signature you'll see on the NEXRAD radial velocity (Doppler) when a microburst is occurring. Green represents hydrometeors moving toward the radar site and red are hydrometeors moving away from the radar. The image below is time stamped at 1850Z, when in fact, the microburst began around 1845Z based on the camera's time stamp.
The dual-node signature is because the radial velocity can only see hydrometeors moving toward or away from the radar site. However, a microburst occurs in all directions, but the radar can't see hydrometeors moving tangential to the radar site. The velocity is essentially zero in these areas, hence the dual-node signature shown above.
Below is the Doppler radial velocity loop of the event. Each frame in the loop is spaced at about 5 minutes apart. You can see how quickly the microburst occurs.