People give feedback in conversation; both positive signals of understanding, such as nods, and negative signals of misunderstanding, such as frowns. How do signals of understanding and misunderstanding affect the coordination of language use in conversation? Using a chat-tool and a maze based reference task we test two experimental manipulations that selectively interfere with feedback in live conversation: 1. ‘Attenuation’ that replaces positive signals of understanding such as “right” or “okay” with weaker, more provisional signals such as “errr” or “umm” and 2. ‘Amplification’ that replaces relatively specific signals of misunderstanding from clarification requests such as “on the left?” with generic signals of trouble such as “huh?” or “eh?”. The results show that Amplification promotes rapid convergence on more systematic, abstract ways of describing maze locations while Attenuation has no significant effect. We interpret this as evidence that ‘running repairs’ – the processes of dealing with misunderstandings on the fly– are key drivers of semantic coordination in dialogue. This suggests a new direction for experimental work on conversation and a productive way to connect the empirical accounts of Conversation Analysis with the representational and processing concerns of Formal Semantics and Psycholinguistics.