Time manipulation. Only thing I can think of, or dominating someone by not letting them do their thing? Which would mean it may also point to defensive efficiencies that are unmeasurable. Defensive coaches do rank highly, while offensive don't, when using PER that way.....
Oh, and yea, PER when used to evaluate teams is very flawed, it's a player ranking and this isn't the best way. You already know that. I only used it because I have nothing else. I trust the talent vs win percentage more. Though 50% is PER based, the rest is a stat based on multiple subjective ranking systems condensed. It's too subjective though.. I guess.
There's no other coach ranking system I know of. When it's done game by game. Well, it means nothing. When it's done in a season, it still means nothing. Five or more years it shows trends, 10 years... 15 years and if the same trends are consistent.. you have information.
We have 17 on Crean, he's been very consistent. Almost like clockwork, with the 10/11 and 13/14 seasons being the only outliers. He drastically underperformed in talent vs result in those 2 seasons. The rest he was barely under the line. His players produce, but his teams don't. He falls just short of ranking almost every year. In PER vs results it's even worse.
Look at this year, which everyone thought was a great job. 5th in talent rating to start, adj to 8th for JBr. 3rd (adj for JBr) conference. So the unbiased expectation was 8th nation, 3rd conference, conference semi, fourth seed tourney, elite eight.