Canon EOS-1D X Mark III Advanced User's Manual page 659

4.
Record the movie.
Movie recording standby
Movie recording in progress
Select (4) Shutter button, and when you release the button, movie
recording begins.
During movie recording, the red [○] on (4) Shutter button changes to a
white [□].
To stop movie recording, select (4) Shutter button again and release
the button.
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