Both Morgan Le Faye, sister of King Arthur and Nyneve, the Damosel of the Lake, are powerful sorceresses, meaning that both have access to supernatural powers. However, while Morgan Le Faye uses her powers for evil—for example, attempting to take Arthur's throne and later to harm him with a coat that would burn him to death—Nyneve uses her powers for the good, in order to protect Arthur.
Morgan Le Faye's ambition and desire to usurp male prerogatives speaks to anxieties medieval male society had about female power. Nyneve, however, offers reassurance that female power can also be "properly" deployed to safeguard men.
Morgan Le Faye poses a direct threat to male power when she steals Arthur's sword, Excalibur, and replaces it with a fake. She gives the real sword to Accolon so that he can defeat Arthur. Morgan hopes then to rule through Accolon.
In contrast to the ambitious Morgan, Nyneve, when she finds out about the scheme and then sees the powerful body of Arthur, feels the proper female admiration and sympathy for Arthur:
When the Damosel of the Lake [Nyneve] beheld Arthur, how full of prowess his body was, and the false treason that was wrought for him to have had him slain, she had great pity that so good a knight and such a man of worship should so be destroyed.
Rather than want to replace Arthur body with her own by having him killed and taking his power, she is properly subservient, using her sorcery to get Excalibur back to Arthur.
She has "love" (reverence) for Arthur, and this causes her to intervene a second time to protect him when Morgan sends him the "gift" of the coat that will burn him up.
Female power is "bad" when it is used by a woman to pursue her own agenda and to gain power for herself. It is good when it is used by a woman who admires powerful men and wants to help them, without having her own rival ambitions.