How could I make this countdown work?

I'm trying to make a countdown start when the RadiusBrick is touched and when it's untouched the countdown starts to go back down. Just like a conquest style gamemode, if a player has captured the node for let's say 5 seconds and the player leaves the capture radius, the countdown decreases from 5 instead of returning to 0 right away. The touching functions work fine from previous tests but I'm not sure what to put in the countdown parts of this script.

Beams = script.Parent.Beams
SkyBeam = script.Parent.SkyBeam
NumValue = script.Parent.Value.Value
Countdown = script.Parent.Countdown.Value

RadiusBrick = script.Parent.RadiusBrick

RadiusBrick.Touched:connect(function(otherPart)
	if NumValue == 1 and otherPart.Parent:FindFirstChild("Humanoid") ~= nil then
		NumValue = 2
		Countdown = 
	else
		end
end)

RadiusBrick.TouchEnded:connect(function()
	NumValue = 1
	Countdown = 
end)

Could someone please tell me what I need to put in the countdown parts or give me some tips?

Thanks.

local NumValue, lastIndex, countdown, debounce do -- creating new scope
	countdown = function(start, stop, step) -- numeric loop function
		local last -- later set to the last `index`
		if not step then -- the `step` argument is blank or nil
			for index = start, stop do -- do not take it into consideration
				print(index);wait(1)
				last = index
			end
		else --elseif if the step is provided
			for index = start, stop, step do -- do take it into consideration
				print(index);wait(1) -- prints each index with interval of `1` (wait)
				last = index -- sets the `last variable` equal to the `last index`
			end
		end
		lastIndex = last -- the `lastIndex` variable outside of this `block`
		--because we can't use the `last` variable created here in other blocks
	end;

script.Parent.Touched:connect(function (hit)
	if pcall(function() return hit.Parent.Humanoid end) and NumValue  then
	--whenever the script's parent is touched, we start the countdown
		countdown(1,5);
		print(lastIndex)  --and print the last index
	end
end))

You could make use of the lastIndex variable when setting the reverse countdown starting from that last value.