Yes the USB host port 5V hole. Not really sure what it's for,
It's for powering up USB devices.... in a glorious but distant future when we have a mature USB host library.
Currently this very early test code is all that exists.
https://github.com/PaulStoffregen/k66_usbhost
The code which turns on the USB host power is in USBHost::begin() in ehci.cpp.
I thought it might supply 5V.
It does indeed do that, or almost 5V, but only under software control. By default it's turned off.
This isn't just any ordinary 5V. The special hardware is meant to support hot-plugging USB devices. Without this circuitry, if you just hot-plug a USB device to Teensy, which is itself a USB device with it's initial hot-plug capacitive loaded limited, the sudden current flow to charge the capacitors in the new device could momentarily drain Teensy's power too low, cause it to crash or reboot.
The power switch is also meant as a measure to allow applications to deal with poorly behaved USB devices. If they go very wrong, it'll always be possible to hard reboot them by power cycling.
New to microcontrollers and electronics, just stepping outside the arduino sphere. Thanks for the help.
Also new to forums? You duplicated this question from
your other thread, in the process annoying one the more helpful regulars here who was trying to help you. A simple rookie mistake, but not the best way to get help...