Colonel Barriffe provides a set of commands for this method (although he does not give a name for it)*.  In essence, two files of musketeers become a single rank.  That rank then fires.  When retiring, that one rank splits back into two files, which go back to stand next to the pike.

Repeated volleys may be had with this method, with the files of musketeers sequentially shifting from inside to outside their sleeves and, once as the outmost files, going forward, ranking inward, and blasting away at the enemy.  By the way, the files of musketeers may advance farther from the pike than is shown in the animation.

Presumably to maintain the positions of File Leaders relative to their files, Colonel Barriffe has the two files after firing, "... wheele off to the right and left; following their file leaders in single files..."  After study of the above animation, some may consider it simpler to have the two files after firing to "face to the outside," thence follow their Bringers Up back to be in line with the pike, and finally to face inward and march forth to their places, thereby returning each File Leader to his position as File Leader.  Although Colonel Barriffe distinctly used "wheele off," he also stated in the margin: "No Commander so precisely bound to any forme but that he may as occasion serveth alter this Figure at his pleasure."**

*Chapt. 73, Military Discipline...
**Chapt 74, pg 83, Military Discipline..."