Teaching his players painfully boring defensive football, making defensive changes when something like that happens and parking the bus. With a full 11 he'll park a bus and try and win in the final 10 minutes in some games.
His first year or two, his team was mostly the uber attacking Chelsea team that was built before he got there, he turned it into the most painfully defensive and dull team I've seen in years given the quality they had there. Chelsea are FAR more interesting to watch since he left and Drogba's gone from an uber attacking high scoring player in his first year, to a defensive lone striker diving at every opportunity to hold up the ball as he waited for support, to again finally this year an uber attacking striker again scoring a heck of a lot more than previous years.
If he came back to the prem, he might win something, he might not, but whatever team he managed would become more defensive, buy some of the best defenders in the world, and be utterly boring to watch.
He's a great manager for a defensive boring team, but its boring football, its worth pointing out that Porto were simply unopposed in their league as they were before he got there, and Inter were unopposed in the league for what, 5 years now, way before he got there. Chelsea he did well, in years where Utd and Arsenal were weaker, but his record there was far more impressive given Chelsea not having anywhere near the league advantage Inter/Porto had. But its not like Chelsea didn't have a fantastic defensive squad before he got there either, Gallas, Terry, Melchiot and, whoever the heck was at left back