Let the jars sit undisturbed while they cool, from 12 to 24 hours. Do*not*tighten ring bands on the lids or push down on the center of the flat metal lid until the jar is completely cooled.
I'm so surprised that none of you have heard of this before that I think you must have misunderstood the question. From the National Center for Home Food Preservation: https://nchfp.uga.edu/publications/uga/using_bw_canners.html#gsc.tab=0
The reason is to allow the jars to cool completely at room temperature without possibly bumping them against something that could break the seal. I have a tray ready lined with a towel and when the processing time is up, I wait five minutes and move the jars to the tray. Now I can move the tray to an out-of-the-way place so they can sit undisturbed for at least 12 hours.
Sometimes you hear some right away. But I've often heard them ping later in the evening when I finished canning late afternoon.
They don't all ping at the same time. Pretty close but not always.