Both of my CO2 systems (keezer and carb fridge) have external tanks & primaries, and I have an extra drop on the keezer system with a QD.
I unscrew the QD, stick the hose into the mouth of the carboy by an inch or so and pointing straight down into the center, open the manifold valve and blast away for a few seconds, slap the airlock on and stick it in the fermenting fridge, and go have a beer.
Like so many things, I don't know that flushing the head space on a secondary fermenter is necessary
, but I do it anyway, especially because all of my carboys are 6.5 gallon glass and I keg in cornies. I've never, ever had an obvious infection in years of brewing.
A good case of OC can be handy for the home brewer, imo...