It's basically a form of peening. In the cases I'm familiar with, it's applied to the weld toes. This basically precompresses the material in that area and makes it less subject to tension cracking.
Keep in mind it's a proprietary process and you'll have to pay royalties (per foot treated or something like that) in addition to buying or renting the equipment. But by all accounts it's much easier to do and more pleasant for the user than conventional peening.
More links:
http://www.appliedultrasonics.com/(Applied Ultrasonics, the guys who own the process--I guess you've been there already)
http://crossroads/org/rti/docs/0-4178-S.pdf(a study done at the University of Texas)
One thing the Texas folks found was that you can't do anything hot to it after you treat it, or you lose the benefits. So no galvanizing, thermal spray, hot-stick galvanizing repair, or heat treatment.
Hg
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