It is pretty much what we think of when motorheads like us read it: a big honkin' accumulator and some way to force the hydraulic fluid into the accumulator as the truck is braking, and then using that stored energy to "launch" the vehicle off the starting line. I saw a little blurb on it in UPS's stock news last fall and didn't think anything of it. UPS has been making some major changes to their brown trucks in the last year. If you listen to one of them, you'll might hear a diesel that doesn't sound like the old International engines they used to have. They're now running (at least here) a 2.5 liter Mercedes diesel and their trucks are getting over 21 MPG on the open road.

Here's another piece of this "HLA" technology which the press release doesn't explain: a new motor that is much more efficient than the fixed or variable displacement vane pumps or motors of old:

http://www.hydraulicspneumatics.com/200/GlobalSearch/Article/False/6527/

If they can make it work, there's some real upsides to it: the hydraulic system is completely sealed, so it might be able to get away with no fluid changes at all. For medium trucks, a significant component of why their fuel usage is so high is stop-n-go driving. Being able to capture the energy of coming to a stop and use it for something other than heating the brake pads is a really good idea for conservation.

WRT batteries: I think that for the size and application of the vehicle, this will save weight over a battery hybrid drive setup.