This video shows and tells, in a general way, about the momentum involved when one person punches another:
[vimeo]66831703[/vimeo]
That advice, with a bit of physics thrown in, is most helpful for taking a punch coming towards one side. The video does not tell what to do if that punch is unavoidably coming squarely at “your center line, including your groin, solar plexus, and throat,” as it is in this video:
Many other people offer their own advice about how to take a punch. Here’s a small selection: