A critical aspect of Mathematics Teacher Leaders' (MTLs) multi-faceted work is facilitating collective disciplinary discussions relevant and accessible for teachers. While research has identified goals for teacher learning, how MTLs learn to support these goals in content-specific ways is still under investigation. In this study, we lift the classroom design and analytic framework of Productive Disciplinary Engagement to examine the work of 73 MTLs and two expert facilitators in a professional development setting. Our use of the framework as an analytic lens to examine leader professional development revealed critical MTL and facilitator practices that supported productive mathematical engagement and practices that hindered engagement. We contend that productive disciplinary engagement provides a fruitful lens as an analytic framework for research. We also argue that it may serve as a valuable design framework for MTL and teacher professional development in future research and development efforts.