It appears that it can be interpreted/applied as follows: If a club buys a player for £100m on a 5-year contract, then they've only paid out £20m!
It will be interpreted that way, as payments will be structured that way. If a club paid £100m up front, it would come out as £100m for the one season. It's football related outgoings vs income each season.
It'll be done exactly the same way the financial accounts are done. Regardless of when the money is paid out the fee will be split over the duration of the player's contract. What will be more interesting to look at is deals where players are sold seemingly cheaply in deals that cost the selling club money compared to what they signed the player for. Because of the way the money works it could be an early warning that a club is struggling to meet the rules and might therefore be an easy target if we want somebody.