There are many different zoom flashlights available, but almost all are "budget" flashlights.
They all work the same way. They have an aspheric lens at the front that works sorta like a magnifying glass, but with a flat back. In spot mode, the LED is placed at the focal point of the lens, producing a focused image of the LED die. To get to flood mode the distance between the lens and LED is reduced. This causes the image of the LED die to defocus becoming a wider, perfectly uniform circle of light.
Most zoom lights work by having the lens mounted on a sliding bezel that can be moved back and forth. Some zoom lights work by having the pill with the LED slide back and forth while the lens remains stationary.
Note that for max throw you need a high surface brightness emitter. The higher lumen emitters like XML and XML2 are very efficient and produce a lot of lumens... but they have low surface brightness. In a zoom light they'll give good flood, but poor spot. Smaller, brighter emitters like XPE2 or XPG2 will produce more lux but less lumens.
Advantages of zoom lights:
1. Produces a perfectly uniform flood with no hotspot. Sometimes this is what is needed.
2. Width of the beam can be narrowed to avoid interrupting neighbors
3. Has more throw (lux) than any reflector light of the same size using the same emitter.
4. Spot mode is more useful for seeing things at long range than the hotspot of a typical reflector light. Unlike with a reflector light there's no spill on things in the foreground washing out light reflected off distant things in the background.
Disadvantages of zoom lights:
1. Due to the lack of a reflector flood mode tends to be much dimmer than the flood mode from a conventional reflector.
2. Flood mode also isn't usually as wide as a reflector light. In some zoom lights the gap between the LED and back of the lens is left too large resulting in a useless flood mode that's like shining a light through a toilet paper tube. This isn't a hard and fast rule though. Many zoom lights have a very usable flood mode. And I've modded a zoom light to have a flood mode that's wider than the flood on most of my reflector lights.
3. Not waterproof. The sliding bezel changes the internal volume of the light. In most cases this means zoom lights are intentionally left not waterproof. This is necessary to allow air pressure to equalize inside the light when the zoom is cycled. If it were sealed external air pressure would force the bezel to return to whatever position it was in when the compartment was closed.
4. Cheap quality. Most zoom lights (including the one you listed) qualify as budget lights with questionable quality. There are only a few brands known to make higher quality zoom lights (LED Lenser, Wolf Eyes, Lenslight, Poplite). The cheap quality lights may or may not work right when used, and do not have coated optics.
5. Inefficient spot mode. In most zoom lights, cycling the light to spot mode means moving the lens further from the LED. This causes a lot of light coming from the LED to be absorbed into the sides of the sliding bezel and be lost. The result is that while lux may go up in spot mode, overall lumens may drop by 50%.
6. Rings. Some lower-quality zoom lights may have unsightly rings around the outside of the beam. These are caused by reflections off the inside of the bezel, pill and lens retaining ring.
Popularity
Zoom lights are very popular as budget lights. The most popular one is probably the Sipik 68. The flood beam isn't that bright, but it has a lot of throw for such a small light. However, smaller and better zoom lights are available.
LED Lenser
LED Lensers have a slightly different optic than other zoom lights. Instead of a simple aspheric lens, they use a "reflector-lens" setup. The reflector-lens is a one-piece moulded plastic comprising a TIR optic at the sides with a small aspheric lens in the center. There's also a hole in the center for the LED to move in and out of. The LED itself is mounted on a post instead of a conventional star. Since the LED is always surrounded by the TIR optic in both spot and flood positions, overall lumen output stays uniform when the light is cycled between spot and flood. You don't get the 50% drop in lumens in spot mode like with other zoom lights.
Modding:
Zoom lights can be fun to mod. Putting in a dedomed emitter can nearly double the throw while reducing flood only a modest amount. Filing down the pill and body to allow the back of the lens to retract closer to the top of the emitter can dramatically increase the width and usefulness of the flood mode.
My current EDC is a heavily modded Aleto N8 zoom light. I cut it down to 91mm, added an electronic sideswitch, bumped up the output with a FET driver and dedomed XML2, and replaced the lens with a flatter fresnel. It runs on an 18650 but is smaller than my Zebralight SC62, making it one of the smallest 18650 lights. It has a powerful flood mode: Over 90 degrees wide - wider than the spillbeam on all my lights except my MTG2 with over 1,000 lumen output. In spot mode it produces 27,000 lux..... not fantastic, but not bad for such a tiny light. The only thing I don't like about it is the tint could be better. It's hard finding a dedomed XML2 with good tint.
