Nexus 7 won't boot after installing twrp In How do I root my Nexus 7? step 7, I couldn't figure out what "on-device fastboot controls" meant. Now when I boot the nexus 7, the google logo with an open padlock appears for a second, and then the colorful 4-circles android animation spins, for many hours.

Just once, I saw the Teamwin splash screen, showing the version number 3.1.1-0 matching the twrp.img that I'd downloaded from dl.twrp.me and flashed in step 6.  (Yes, it's the right version, "flo" for nexus-7 2013 wifi.)

adb devices and adb reboot bootloader both report no devices found.

The battery is full, and staying full while the USB cable to the PC is plugged in.

How do I restore the tablet to a state where I can either give it commands from its screen, or from adb over USB from my win7 PC?

Is there some magical button-holding or adb command that can interrupt or prevent its infinite loop?  Holding the power button for 5 seconds returns it to the google logo, but then the dance starts all over again.  (It's fine to wipe the data on it. My eventual goal is to install cyanogenmod CM13.)
Partial answer
On-device fastboot controls means using the hardware buttons as explained in How to enter Google Nexus 7 Fastboot Mode.


Power off the device completely.To enter Fastboot mode, hold down Volume Down and Power.

Release the buttons only after the Fastboot screen appears, which shows the green robot on its back with its cover open.