Canon EOS RP Advanced User Guide - Page 228

Setting the Flash Function zzShutter synchronization Normally, set this to [First-curtain synchronization] so that the flash fires immediately after the exposure starts. If [Second-curtain synchronization] is set, the flash will be fired right before the shutter closes. When this is combined with a slow shutter speed, you can create a trail of light such as from car headlights at night with a more natural feel. When second-curtain synchronization is set together with [E-TTL II flash metering], the flash will be fired twice in a row: once when you press the shutter button completely and once right before the end of the exposure. If [High-speed synchronization] is set, the flash can be used at all shutter speeds. This is effective when you want to shoot with background blur (open aperture) in locations such as outdoors in daylight. ooWhen using second-curtain synchronization, set the shutter speed to 1/80 sec. or slower. If the shutter speed is 1/90 sec. or faster, first-curtain synchronization will be applied automatically even if [Second-curtain synchronization] is set. 228