Position–velocity diagram for HCO⁺ emission along the DR21 Main outflow. The offset is measured from the edge of the eastern lobe towards the west. The arrow points to the 9 km s⁻¹ absorption feature (Dickel et al. 1978), the white arrow highlights the location of the interaction region (this work), the magenta dashed contour marks the H41α emission from the Η II region, and the light green arrows mark examples of sawtooth pattern structures (Santiago-García et al. 2009).