Page 69 - Nate Fitch, Ron Funderburke "Climbing Knots"

the initial slipknot functions best when the attachment
carabiner is an adequately sized HMS or pear-shaped
locking carabiner.
If tied correctly, the mule knot tends to constrict
upon itself and will break in front of the initial slip-
knot only at very high loads. As a result, it is a reliable
and secure way for a belayer to secure a climber when
the brake hand is needed to perform some auxiliary
task.

Mule knot for a
plate/aperture
belay device.

Carefully pull a
bight of the brake
strand through
the locking
carabiner.
56 CLIMBING: KNOTS