Canon EOS-1D X Instruction Manual page 356

85: Custom Controls
: Start movie recording (when k set)
With [LV z/k set.] set to [Movies], pressing the button assigned with
this function will start the movie shooting immediately. To stop the movie
shooting, press the button again.
: Switch to Custom shooting mode
When the shooting mode is not <w1>, <w2>, or <w3>, you can press
the <B> button to switch to the registered Custom shooting mode
(p.361). If multiple Custom shooting modes have been registered, each
time you press the <B> button, it will switch the shooting mode from
w1 → w2 → w3 → current shooting mode. However, during movie
shooting, pressing the <B> button will not switch to the Custom
shooting modes. (Movie shooting will start.)
: Image replay
Press <0> to play back images.
: Magnify/Reduce (press SET, turn )
Press <0> to magnify the images recorded on the card. See page 259
for the operation procedure. You can magnify the Live View image
during Live View shooting and movie shooting when focusing in Live
Mode, Quick Mode, or with manual focus (p.218, 222).
: No function (disabled)
Use this setting when you do not want to assign any function to the
button.
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