The Big Zip of the name relates to the opening of the bladder that is across the width of the top. There’s a closure similar to a ziplock bag, and then a plastic ‘SlideLock’ that ensures it is leak-free. I like the handle on the top. It’s especially useful for carrying water from a stream at camp, as well as serving to attach to rucksacks. The wide opening also makes the main reservoir very easy to clean, although a cleaning kit is available as well. It has an anti-microbial treatment, but there is a slight taste as well. The bite valve is good and there’s an excellent flow. The water is easily locked out too. An insulated version, with a case made from Cordura, is available that insulates the entire pack and hose, although I think only the hose needs insulating. Unlike the Osprey reservoir, the hose can be easily detached while there is still water in it. It’s an excellent system, except for the bulge, which can be uncomfortable in some backpacks.