Hard restart on iPad

Today, for the first time ever, an iPad completely froze on me.

Usually, if one holds down the Wake/sleep button (on the edge), one will first get a slide option to shut down the iPad... and if one continues to hold down the button, the iPad will shut down regardless.
I thought that was a "hard reboot", meaning it bypasses the software. But this time it didn't even respond to that.

But then I thought of trying something which has worked on similar devices: holding down the Sleep button and the Home button at the same time for a few seconds. Boom, that worked, it shut down, and worked again when I started it.