Hah! Thank you very much! I'm an American-born native English speaker but I didn't know what the phrase meant. I looked it up in the Urban Dictionary http://www.urbandictionary.com/define.php?term=take%20a%20knee There is the literal meaning from American football that you found in which the quarterback gets the ball and immediately puts his knee to the ground to end the play. In effect, the whole team gets to take a break from play without taking a timeout and without risking a "delay of game" penalty. So... that's the literal meaning. The figurative meaning of "take a knee" is "take a break from whatever activity you're doing and get some rest". That's why Ryan sits down. This is not a common saying except perhaps among American football fans.

Generally, soldiers kneel on one knee when at a semi-rest since this is a shooting position but comfortable. No, in common usage you cannot say that. It is basically only informal and if outside for normal people, or in the case of soldiers, anytime in a combat zone. But note, that actually, he should not have sat down technically. He wasn't actually listening very well :)