Portable washing machines work a little differently from one another, depending on the design of the manufacturer. Some work by hooking up to an existing faucet of some kind, filling up the chamber through a hose, and then draining through a second hose once the process is done. Others work by allowing manual filling of the chamber with water, and use a simple drop-down hose to drain into a bathtub. These work great if you have a detachable shower head, making them easy to fill up.
Surprisingly, some of these compact washers operate much faster than a typical washing machine, finishing the cycle in just a few minutes. This may sound strange, but remember that this excludes the initial process of filling up the unit with water, which takes a lot longer with typical built-in units. Also, some portable washers skip the rinse cycle, since after draining the water, they must be filled manually again, after which they can be run again without detergent to rinse the clothing.
After finishing, the clothes can be hung up to dry. Keep in mind that some portable washing machines have no spin cycle (the final process of a typical washing machine, which leaves clothes damp instead of dripping), which makes hang drying slower, taking up to 24 hours or more. This can be helped by wringing out the clothes, or buying a separate spin dryer, or finding a portable washing machine with a built-in spin cycle.
That said, here are some of the best compact, portable washing machines for apartments, all of which get rave reviews, particularly the Panda Portable Washing Machine, or check out Wonderwash and Laundry Pod for hand powered washing machines for power outage use: