Basics:
Force Light weakens the victim's ability to use the dark side.
Anything that the victim benefits from via the dark side (mind control, powers, strengthened body) is temporarily lost or weakened (or permanently so, if the victim's Writer truly wishes to do this).
With the concept of matching a Writer's effort in kind, this temporary weakness might be brief (if the user of Force Light just initiates and executes in the same post) or long (if the user has been prepping for several posts before executing). Though, it usually just depends on how long the victim's Writer feels like playing with the imposed weakness.
It also really depends on the victim's traits, too. Darth Junra had spent years without using the dark side anywhere near Jedi. She had become used to fighting without using the Force's influence, and thus Force Light wouldn't be as devastating as, for example, a Sith that required it to live.