That's a good link Larry, thanks.
That's actually an 'old' technique now -- at least to me it is.

But it's become one of the best methods used now though. I highly recommend it.
If you want to get "creative" with your sharpening/blurring, try using Curves on the channel to create the mask. Or, try also using the "Trace Contour" filter, then apply either the Maximum or Minimum filter to the channel -- which one you use depends on the bg colour of your channel.
And... you could also apply some of the other filters to create an even more interesting selection to work with -- like the filters under the Artistic menu.