Siphons are the most pervasive methods of emptying the water from a media bed. Generally, a siphon works by establishing a pressure difference between the water-filled section of the media bed and the end of the siphon tube leading into the reservoir or fish tank. To work properly a siphon must first "engage" which means to fully establish this low pressure vacuum that will suck the water out. A common example of this is to use a tube that's submerged in water so that no air bubbles exist. If you were to take one end of the tube and lower it so that it is lower than the end that serves as the water intake, the water would naturally siphon out, naturally getting pulled by the collective pull of low pressure similar to what happens when you drink through a straw.
There are many configurations of piping that can do this but in my experience the bell siphon is the most popular. Bell siphons have some good benefits. They pull from the bottom of a media bed, ensuring that most of the water is expunged but also they tend to concentrate bottom sludge into one point and get clogged. Bell siphons are great if your input flow rate is very small as you can reduce the bell siphon standpipe quite significantly to work in low flow rates (more than 20 minutes to fill media bed). They're fairly easy to engage with proper diameter-reducing funnels to decrease the amount of water required to fill up the downstem and thus start the pressure differential. They're fairly easy to make. But what if I told you that there's something that's easier.
U-Siphons, as you can now tell, are my favorite way of siphoning media beds. They're much more simple to make, incredibly easy to install and tune, and engage consistently as long as the pump input is less than the siphon output when it still has not quite engaged. U-siphons are also completely external from the media bed so they do not take up valuable growspace and they're very easy to modify and troubleshoot as there is no excavation required if it were to get clogged or needed to be removed for maintenance. Additionally, you have the options of placing it on the side of the bed with a slitted intake pipe that extends the length of the media bed and takes in water from many points, not just one as the bell siphon does.
All in all, each application has it's appropriate solution but next time you find yourself looking for a way to empty your grow beds, consider a U-Siphon.