Underflow is when a flow front reverses direction.
It occurs when flow fronts from two directions meet, pause momentarily, then one of the flows reverses direction and flows back between the outer frozen layers. When the flow reverses direction the frozen layer partly re-melts due to shear heating.
In the animation below, the gate in the center has a much smaller volume to fill than the other two gates. When the center gate's volume is filled, the other volumes are still filling, so the flow front from the left gate has a lower pressure than the center gate. When the two flow fronts meet, the left flow front reverses direction.
The arrow shows the direction of the underflow.
This flow reversal gives poor part quality, both from surface appearance and structural viewpoints.
What to do
Inspect the filling pattern to assess if underflow is likely to occur. Displaying the fill time result at 100% will not indicate the presence of underflow. Play the fill time animation from beginning to end and watch for any flow fronts meeting, then consider the geometry surrounding this point.