In some enlargers, the negative pops after the lamp heats up. One way to avoid this is to leave the light on and use the red filter as a shutter. To prevent enlarger movement while swinging the filter under or away from the lens, hold the other hand under the lens while moving the filter, and control the exposure with that hand.
I always wondered about the grain focusing part, you do that BEFORE the paper is added, FB is thick, this would seemingly "throw off" the "perfect" grain focus, and the red filter would alleviate this by allowing you to focus once the paper is laid down.
Am I the only one who ever wondered if the focus would be off since the paper wasn't down when you're using the grain focuser?