Colonel Barriffe does not provide specific commands for this method, only a description (and he does not give even a specific name for it)*.  The core of this is the frontmost musketeer in the file firing, then countermarching to the rear of the file.  As presented by Colonel Barriffe, this method will have only two musketeers at a time forming a rank - at opposite ends of the formation - and firing.  By the way, the files of musketeers may advance farther from the pike than is shown in the animation.

As you may be accustomed to the concept of a long rank (or two) firing simultaneously in a volley, this popping off a mere two shots at a time may seem an exercise in small thinking.  Colonel Barriffe mentions, though, this particular presentation of musketeers may be useful: "for the beginning of skirmishes... or to provoke the Enemy to battell, to draw them into some ambushment and other such politicall ends..." So, this is done in anticipation of bigger things.

Its simplicity makes for a good example of files of musketeers arrayed with pike and coordinating movement to engage the enemy, thus an introduction to more complex examples.

*Chapt. 73, Military Discipline...