Canon EOS RP Instruction Manual page 436

Setting the Flash Function
Shutter synchronization
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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.
When using second-curtain synchronization, set the shutter speed to
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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.
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Normally, set this to [First-curtain
synchronization] so that the flash fires
immediately after the exposure starts.