The manual CO2 injector is a push style button that releases gas from the 16g CO2 bulb into the keg. It is what the complete picnic tap package comes with as standard.
After a couple of goes they are actually quite easy to use and are also useful for flushing a mini keg or growler with CO2 prior to filling with beer.
Use them in very short bursts, it is easy to over pressurise a keg using this if you press too long.
The outlet is M8 thread and needs an adapter if you want to use it on an MFL disconnect