The Premier league has FFP based loosely on the UEFA model. It stops you increasing wages by x amount per annum. That alone would have saved Leeds and Portsmouth. Leeds wages increased by nearly 50% in 2001, that couldn't happen now.
So basically, if you are already rich & have a huge income due to years of Champions League exposure & time spent near the top of the Premier League, generating money directly from CL tv payments but also by selling merch to the people around the world who have seen you pushed on tv every week, then you can pay Alexi Sanchez 500k pw to sit on his arse, & your wage bill is already so high, the 'extra' 500k pw, doesn't even push it up by a significant percentage.
So Utd are there, with a wage bill of over 300 mil, & along come Leeds for example, a club with great 'history' who have played in European Cup finals.
But they are struggling to finance a 30 mil wage bill.
But let's imagine it's no problem, because they are lucky enough to be taken over by a relative of Sheikh Mansour who is willing to finance them, same as City were in the early days of our takeover. They can try to spend, to bridge the gap to the teams who have been coining it in due to 20 odd years of Champs Lg finance.
Ah but, even though Leeds uncover 3 or 4 future stars from around the world at 10 mil each, & make some brilliant signings, at a great price, for other positions, it puts their wage bill up to 45 mil.
So they can just fuck off. Not allowed.
Whilst Utd spunk away money on Sanchez, Pogba, Lukaku, any one of which costs more than the whole increase Leeds, as a club, are allowed to make.
So each year, Leeds try to get that little bit closer by increasing their wage bill a little & each year Utd etc, spend even more increasing their wage bill by more than Leeds & get further away & if Leeds do get a top player, Utd etc steal him by persuading him to push for a move, so get him cheap (a bit like Bayern Munich do).
So 40 years later, Utd, with American owners fleecing the club, are still able to pay the next Sanchez 4 mil a week & Leeds, with an owner who wants to put money in, are still not able to pay the wage bill Utd are paying now.
Welcome to Financial Fair Play, a system designed to stop clubs who are not already rich, from competing with those who are.