Imagine if you have hot(+) bricks. If you add 2 hot bricks with 2 other hot bricks, you will have 4 hotness in total. Now let's say you have some cold(-) bricks. If you have the 4 hot bricks and add the two cold ones 4 + (-2) = 2 The cold bricks substrated from the overall hotness . However if you took away these cold bricks 2 - (-2) = 4 When you take away the cold bricks you bring the hotness back up. Hope this helps.
