I'm not sure how old you are or if you remember waterbeds or not but it basically uses the same technology (not as new as some people think). You can buy all the parts at most home improvement stores and make your own if you want. It attaches to the faucet and you flip the end around, opening the valve, so that it pulls water from the tank. Keep in mind that all the while, you are running your tap water wide open so you are wasting a lot of tap water. That is where the suction comes from. Once you have finished removing water, you simply turn the end to close the valve and start refilling.
I use the pythong to fill my tanks but not to remove water because of how much it wastes. I simply use an old fashioned hand siphon/vacuum (the python also as a vacuum to clean the bottom) and a bucket to remove water. One thing to keep in mind if you do use it to take water out is that depending on your plumbing, the drain may not be able to handle all the water that is going into the sink (your tp wide open plus what is coming from the tank) and will run over if you aren't careful. It's best to pull the end from the tank every so often to let the drain catch up. I speak from experience as I just happened to look the first time I used mine and I was about 1/4" from running over into the floor. And then my wife would have killed me and I wouldn't be on here posting.