For loops going backwards with subtraction?

I can't help but wonder if for loops can go backwards. Let me show an example with my code here.

part = script.Parent

function sum(x, y, z)
	for i = 1, 10 do
		part.Position += Vector3.new(x, y, z)
		wait(0.01)
	end
end

while true do
	sum(1, 0, 0)
	sum(-1, 0, 0)
end

I figured that if this for loop reaches its end/max value, it will stop and the while loop will call the next function and the for loop will go backwards by subtracting. Is this true? To be specific here, I thought for loops could only go forwards, because when I remove the second function call "sum(-1, 0, 0)", the for loop will keep going non-stop. I don't get it, I need to understand how this works. And sorry, I'm new to programming.