Power is about managing conflicts, being able to control yourself while going through one is a huge advantage. Being able to evaluate your options, using the right amount of time for decision making, thinking through what you are actually going to say before starting to speak, these are all skills that depend on self-control.

Yes, it's not 1:1 power, but as the submission title states, it signals it. (Hence the movie cliches about extremely cool guys, don't even flinch while an explosion is going on, and the memes about how millenials flinch when the phone rings, etc.)