I think it's the part where other European teams can still buy our players.
Say your Barcelona, Real Madrid, Bayern etc, and you fancy a player like Dele Ali. You wait until our window shuts and then you buy Ali, leaving Tottenham unable to strengthen after losing one of their best players.
For me that's a massive flaw, easily sorted by bringing in rules so we can't sell either when our window shuts.
THEY COULD JUST NOT SELL HIM