With growing enrollment in computing, undergraduate teaching assistants (UTAs) play a critical role in managing large courses and supporting student learning. Yet, little is known about how UTAs actually teach students. To address this gap, we conducted an ethnographic study of seven UTAs at a large public university, observing 37.5 hours of office-hour interactions (53 sessions) and conducting three hours of follow-up interviews (eight sessions). Our analysis identified six core pedagogical practices: asking open-ended questions, direct editing, prompting, referencing resources, explaining, and expressing affect. We use these practices to build our pedagogical model, showing how UTAs adapt in real time to balance efficiency and student independence. This work guides UTAs, informs training design, and advances computing education research with a framework for pedagogical decision-making.