It can certainly be proactive once our position has been compromised. We don't need to wait for the opponent to punch us. It is just like paak from the elbow to clear the line when we can't use full range. Also, if our man-sau has lost the line, depending on position, bong+wu should be able to respond to that, as we train in laap-sau drills. But again, it's not contingent upon arm contact or passive in fighting. The stimulus to act is our own bodily awareness of vulnerable position. We should act appropriately before our opponent does, or we will end up further "in the past".