Second Curtain Sync: EF 70-200mm f/2.8L IS III with Flash
First curtain sync fires the flash at the beginning of the exposure. Second curtain sync fires it at the end. The difference, on a stationary subject in a dark room, is invisible. The difference, on a moving subject with any ambient light at a shutter speed slower than 1/60s, is the difference between a motion blur that trails behind the subject and one that leads in front of it. The first looks like the subject is moving backward. The second looks like physics.
The EF 70-200mm f/2.8L IS III is the right lens for this because it gives you working distance. At 70mm from ten meters, you have room to see the subject moving across the frame rather than toward you. Motion blur at slower shutter speeds — 1/15s, 1/30s — traces a path. Second curtain sync captures the subject sharp at the end of that path, with the blur as a tail. The motion is legible. The subject is not ghosted.
Set this up on the R5 or R6 via Custom Functions > External Speedlite Control > Shutter Sync > 2nd Curtain. The camera will enforce a shutter speed ceiling at the sync speed (1/200s on most R-series bodies); second curtain requires going slower than that anyway, so the limitation is not relevant in practice.
The exposure balance is between ambient and flash. Increase shutter speed (slower) to intensify the ambient blur; increase flash power or move closer to sharpen the flash-lit end of the motion. You want the blur readable but the flash exposure clean. Shoot in manual mode for both camera and flash. Auto modes will fight you.
The IS in this lens helps with your camera stability at the slow shutter speeds this technique requires, though it will not stabilize the subject. The subject’s motion is the image.
This is not a technique for every situation. It is a technique for situations where showing movement is more honest than freezing it.