Constructing causal mechanistic explanations of observable phenomena is a key science practice that is often challenging for students as most mechanisms involve interactions of unobservable entities and activities. In this study, we examined how gesturing with a computer simulation that depicts the molecular mechanism of thermal conduction supported middle-school students in constructing causal mechanistic explanations. We designed a gesture-augmented computer simulation in which students were cued to use hand gestures to control the simulation. These cued gestures represent core causal interactions of conduction and they prompt students to physically engage with the simulation in conceptually meaningful ways. In this study, we examined how 21 students used the simulation and explained thermal conduction in a semi-structured interview, followed by a mixed-methods analysis. Quantitative analysis shows that students moved toward articulating the canonical causal mechanistic explanation of thermal conduction using the simulation. Three representative cases were identified to explore how students' explanations were facilitated by cued gestures. The analysis shows two main ways the cued gestures supported all students in the study: (1) by helping them attribute causal agency to molecules rather than an entity called Heat; and (2) by reifying the core mechanism of molecular collisions in conduction. Furthermore, the case studies show how each student's unique ways of sensemaking impacted their gesture use. Implications for instruction with gestures and design of augmented environments are discussed.