There are quite a few different versions out there, they all work on the same principle however. If you've ever seen a gel insulin cooler it's exactly the same as that, only HUGE
They claim:
"Using water absorbing, polymer crystals, sewn into supplex fabric, the evaporative process allows heat to physically transfer from the dog to the jacket, similar to our bodies own manner of cooling through perspiration. Keeping your dog cool and protected from heat exhaustion."
I thought it was rubbish at first but with the never-ending summers out here I had to try something and we've never looked back. Nadia doesn't usually like jackets/raincoats in the winter but the cooling jacket in the summer is her favourite thing in the world. We had a different model before but I made the mistake of leaving it on her when I went out one day and she chewed it - I won't make that mistake again..