Harbor Freight sells two types of foot switches. The Momentary Foot Switch (Item 96619) works like a sewing machine pedal - press it to make the machine run and release it to stop. The Power Maintained Foot Switch (Item 96618) works like a regular switch - step on it to turn on and step on it again to turn off.
I purchased the Momentary Foot Switch for my scroll saw, because it seems very similar to the use of a sewing machine. The switch works great for this purpose and I highly recommend it. I also recommend this switch for a drill press, sander, and even a router table where the router switch may be difficult to access. My only complaint is that it's made of plastic.
I don't see myself using a Momentary Foot Switch for larger tools, like the table saw, band saw, or lathe. I move around and change my stance a lot when using these tools, so keeping my foot on the pedal would be cumbersome. Also, the thought of accidentally shutting off power to a tool in mid-cut is kind of scary. The Power Maintained Foot Switch may be a better option for these larger tools. It could be useful as a secondary stop switch or for hands-free operation when handling large sheets of plywood, but I think the switch itself and the power cables could be a trip hazard.
There are some safety considerations when using foot switches, so make sure you follow the safety guidelines that come with the switch. If you have kids, pets, or frequent visitors in your shop, it's a good idea to turn off the tool's main switch when it's not in use and not rely solely on the foot switch. If you blow a fuse or trip a breaker, turn off the tool's main switch before resetting the breaker. Use common sense.