Minecraft wiki entry for Comparator_update_detector, associated text: This method is often used in BUD's. The player can turn any repeater-based BUD into a CUD by replacing the repeater with a comparator. They can redirect redstone with a detector rail, redstone block or a solid block, blocking 2 redstone diagonal to each other. Again, the comparator doesn't realize that the redstone isn't pointing the right direction anymore and it will stay turned on until it receives an update.