This 5-year mathematics professional development project involves 27 elementary teachers prepared and supported as Elementary Mathematics Specialists (EMSs) in high-need, urban schools. Our inquiry centers on their experiences in the EMS preparation program and the development of important outcomes, specifically productive beliefs and teacher leadership. Data were gathered at the end of Year 1 via surveys of mathematical beliefs, coaching beliefs, and coaching practices, as well as individual and focus group interviews. The findings provide insights into participants' growth and work as a "more knowledgeable other," illuminating specific emphases of their efforts and influences of the project. When it comes to mathematical beliefs, as participants shifted toward embracing cognitively-oriented pedagogical beliefs, they became more efficacious in this learner-centered pedagogy. The findings also show variability in specific mathematics coaching practices, including those most participants were and were not using, and areas for improvement. Several themes are evident in the interview data: "growing confidence for the uncomfortable," "creating and finding spaces for stepping up," "advocating for learned mathematics pedagogy," and "building teacher capacity through interactions."