The highly-maneuverable flight of insects and birds provides inspiration to scientists and engineers for the design of autonomous, bio-inspired "micro air vehicles" or MAVs. As a community, we are uncovering the physics of how animals achieve this phenomenal flight performance, so we can create MAVs with similar capabilities. The development of agile MAVs depends strongly on our understanding of the aerodynamic interactions of a flapping wing. In our area of fluid dynamics, the complex 3D flow that results from continuous flapping is not well understood, largely because it is difficult to obtain a complete detailed picture. Our dye flow visualizations of the vortex structures (swirling flow) formed during wing stroke-reversal allow us to identify "key players" in the propulsion when the wing has to turn around. We can then observe how these flow structures change when we alter the timing of the wing motion, which provides insight into the individual contributions of vortices to the overall lifting force produced.