Define the What, Not the How
The good PM crisply defines the target: the what. The how belongs to engineering. This division of labor is the working core of the job. When the PM specifies implementation details, they do the engineers' job badly. When they leave the what fuzzy, the engineers do the PM's job badly.
Crisply is the load-bearing word. A fuzzy target produces a product that ships late and hits nothing. Horowitz also warns about the failure mode on the other side: bad PMs let engineering teams 'solve the hardest problem' rather than the problem customers will pay to have solved, and define products that can't actually be built.
Good PMs define products that can be executed with the team and time available. Ambition without executability is just a slower way to fail.
“Good product managers crisply define the target, the 'what' (as opposed to the how) and manage the delivery of the 'what.'”
Key idea
Own the what, leave the how to engineering, and make the what so crisp there is nothing to interpret.