This unusual-looking tool is a fireman's hose clamp, also called a Wildland hose clamp, named for its first manufacturer (like "Crescent" wrench).

It's used to pinch off the flow of a pressurized hose so that you can change nozzles, or replace the nozzle with a coupling so you can add an extension hose. Here's an example of how it's used (I've cued up the video below to the right moment):

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I'm not sure how long the tool has been around, but firefighting equipment manufacturer C&S Supply reckons it was due for a redesign:

"After years of being forced to use wildland hose clamps that smash knuckles, constantly break and are awkward to operate, C&S Supply has designed a new Wildland Hose Clamp. Offering a much more rigid design, the Clamp is capable of withstanding the most brutal and rough operations. Yet, it is user friendly enough to quickly accomplish the task at hand."

The closed-loop handle is large enough to fit a gloved hand into, and prevents your knuckles from getting smashed. The clamping area is curved, which the company says does a better job of collapsing the hose. They also point out—I totally missed this—that the bottom handle is a tad longer, so that you can step on it "to assist opening while on a charged hose line." Lastly, it's painted in a high-visibility color.