Asked to shake hands, a patient makes a series of tentative, incomplete movements toward the examiner's hand, repeatedly starting and stopping without ever actually completing the handshake, appearing genuinely caught in conflict about whether to carry out the action. What is this?

This is ambitendency: a series of tentative, incomplete movements toward an intended goal-directed action, here a handshake, without the action ever being completed, with the overall impression of genuine conflict about whether to carry it out. It is not negativism proper, of which ambitendency can be regarded as a milder variant, since negativism proper would show more straightforward non-compliance or reflex opposite action rather than this visible hesitation and partial movement. It is not obstruction, which typically leaves the body briefly fixed in an intermediate position afterward, unlike the continued tentative movement described here. And it is not waxy flexibility, which involves sustained, moulded posture on passive movement, not this active, hesitant back-and-forth pattern.