Never aim your flashlight directly into the eyes of an oncoming driver. Visual recovery from bright lights at night can take several seconds. A driver needs to perceive and react to what is going on. If they are blinded by light, the eye has to recover before they can perception and react.
Perception and reaction time is typically around 1.5 seconds. During that 1.5 seconds the vehicle is still moving. Add on the distance necessary to safely stop, plus the distance the vehicle will travel during the driver’s recovery from being temporarily blinded by the flashlight and you can quickly find yourself or others in harm's way.
This is especially important during DUI checkpoint work or high volume traffic control situations like college and pro ballgames where drivers may be impaired. That impairment can increase recovery time from the bright light and overall perception/reaction time.
You should always use a flashlight cone when available and constantly move the light, giving direction in clear, predictable movements.