This is a VERY short post to describe how "I" connected permanently to the underside pins on my Teensy 3.1's
access to the extra pins on the bottom of the teensy, I used a dual row
header, and bent the inside pins at a 90 degree angle, trimmed, and
soldered. That's it. Easy peasy!
This shot shows the Real Time Clock crystal soldered into place.
There's some further discussion on the subject here...
There are also breakout boards designed to give you easy access to these pins...