The principle
If you are unsure of a course of action, do not attempt it. Your doubts will show in the execution. Timidity is dangerous: better to enter boldly. Mistakes are corrected by more boldness.
The application
Half-commitment is the most expensive position in any system. Either commit fully — voice, posture, plan — or do not start. The half-step invites the resistance the full step would have flattened.
The defensive tell — how to spot it being run on you
When someone pushes you to commit "just a little" to a plan you do not yet believe in, decline. Either commit fully or hold. The half-yes will be cited against you later as if it had been a whole yes.