Feedback that changes behaviour
Most feedback training teaches a formula. Formulas are transparent and people resent them.

The problem with feedback in most organisations is not technique. It is that difficult conversations get postponed until they become formal, at which point they are no longer feedback, they are a process.
Timeliness beats phrasing
Imperfectly worded feedback within two days is more useful than a polished version in six weeks. By then the person has repeated the behaviour, others have noticed, and the conversation now carries the weight of accumulated frustration.
If you train managers on one thing, make it speed.
Describe, then say what it cost
Two components. What specifically happened, in observable terms. And what the consequence was, for the work or for other people.
In the client review yesterday, the pricing section had numbers that did not match the proposal, and the client spent twenty minutes on that instead of the scope. That is complete. It does not require an adjective about the person.
Then be quiet
Most managers explain for too long because the silence is uncomfortable. Ask what they saw, and wait. Roughly half the time you will find a cause you did not know about, which changes what the right fix is.
Agree one change, not five
One specific, checkable change with a date. Then check on the date. Feedback that is never followed up teaches people it was performative.
For managers who avoid it entirely
Avoidance is usually fear of conflict rather than lack of skill. Some things that help: a standing weekly one to one so no conversation requires special scheduling, a norm that feedback is routine rather than exceptional, and their own manager modelling it.
And be honest about the incentive. In a lot of companies the manager who raises a problem creates work for themselves, while the one who stays quiet suffers nothing until the person resigns. Fix that and the behaviour follows.