In case you forgot, Barcelona is currently in the midst of a transfer embargo for the illegal transfer of ten minors over a four-year span. Last year, I examined the convenience of a transfer ban imposed in the middle of an open transfer window when the richest team in the world is involved. Fast forward to today, middle of the summer transfer window (the last transfer window before Barcelona’s penalty is lifted), and the club has already been busy. Barcelona signed right wing back Aleix Vidal from Sevilla for a deal worth up to €22 million with variables, and attacking midfielder Arda Turan from Atlético Madrid in a deal worth up to €41 million with variables. You may be asking yourself why these transfers can happen if a club is under a transfer ban, but the real ban isn’t exactly from bringing in new players, it is the ability to register them as a member of your team and have them eligible to participate in competitive matches.
Every player that is starting with a new club must be registered with FIFA under Chapter III of FIFA’s Regulations on the Status and Transfer of Players. Players must be registered either as an amateur or a professional (professionals have written contracts and are paid more for footballing activity than the expenses he/she effectively incurs), can only be registered with one club at a time, and must be registered during one of the two annual registration periods per that league/association (the transfer windows). Per Article 11 concerning unregistered players:
Any player not registered at an association who appears for a club in any official match [emphasis added] shall be considered to have played illegitimately. Without prejudice to any measure required to rectify the sporting consequences of such an appearance, sanctions may also be imposed on the player and/or the club. The right to impose such sanctions lies in principle with the association or the organiser [sic] of the competition concerned.
This is the true power behind a transfer ban; clubs that have competed with unregistered players in the past have faced disqualification from competition, point deductions, fines, and additional prohibitions on registering players.
What FIFA doesn’t do is tell clubs how to spend their money, or what employees to hire. Barcelona could spend €41 million on a janitor for all FIFA cares, as long as he doesn’t play in La Liga, the Copa del Rey, the Champions League, etc. Nor does FIFA say who can practice with a team; former players or free agents go practice with their old teams all the time; no different from Turan and Vidal walking onto the field for training. As long as the two don’t play in a real match, they can essentially be a Barcelona player in every other way.
 Something you don’t often see in transfers: For Arda Turan there was a sell-back clause that expired on July 20, two weeks after the transfer, so the new president of the club (elected on July 18), could have opted out of any deal if he wished.
 Note that unregistered players may not participate in official, competitive matches. They can still play in exhibitions and friendlies.
 Well, apart from the embattled Financial Fair Play Regulations.