Canon EOS R10 User Manual page 164

Shutter sync.
Normally, set to [1st curtain] so that the flash fires immediately after shooting starts.
Set to [2nd curtain] and use low shutter speeds for natural-looking shots of subject motion
trails, such as car headlights.
Caution
When using second-curtain synchronization, set the shutter speed to 1/30 sec. or
slower. For shutter speeds exceeding 1/40 sec., first-curtain synchronization is
used automatically even if [2nd curtain] is set.
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