Power Drainage Part 2?

It keeps going down when you let go of the CTRL key because you have nothing that breaks the loop that lowers power. In order to break the loop, you have to have a break command in there. Like so:

mouse = game.Players.LocalPlayer:GetMouse()

repeat wait() until game.Players.LocalPlayer.Character["Torso"]

L = Instance.new("SpotLight", game.Players.LocalPlayer.Character["Torso"])
L.Brightness = 8
L.Range = 25
L.Enabled = false

mouse.KeyDown:connect(function(key)
    if string.byte(key) == 50 then
        L.Enabled = true
        while true do
            if (not L.Enabled) then
                break --This breaks the loop if the Light is disabled
            end
            game.Lighting.PValue.Value = game.Lighting.PValue.Value - 1
            wait(3)
        end
    end
end)

mouse.KeyUp:connect(function(key) 
    if string.byte(key) == 50 then
        L.Enabled = false
    end
end)

While loops would be very ineffecient for this. I would use a repeat loop for this. You could make a boolean variable that checks if the key is held down or not, and then at the conditional statement, you could put if the boolean is true or not.

Check this out:

mouse = game.Players.LocalPlayer:GetMouse()

repeat wait() until game.Players.LocalPlayer.Character["Torso"]

L = Instance.new("SpotLight", game.Players.LocalPlayer.Character["Torso"])
L.Brightness = 8
L.Range = 25
L.Enabled = false

keyHeld = false --This would be the variable to check if the key is held down or not.

mouse.KeyDown:connect(function(key)
    if string.byte(key) == 50 then
        L.Enabled = true
        keyHeld = true --Sets keyHeld to true.
        repeat game.Lighting.PValue.Value = game.Lighting.PValue.Value - 1
        wait(3)
        until keyHeld == false --The loop will repeat itself until keyHeld is false.
    end
end)

mouse.KeyUp:connect(function(key) 
    if string.byte(key) == 50 then
        L.Enabled = false
        keyHeld = false --Sets keyHeld to false.
    end
end)

Hope I helped. :)