quote below is from the Maintenance Manual. (a 747 manual, but you get the picture)
My highlighting.
C. Prepare for Engine Operation
(1) Check that airplane is parked in clean area with wheels on areas
that are free of oil, grease, or other slippery substances.
(2) Make sure the wheel chocks are installed at the main landing gear
wheels and ground locks are installed.
(a) Do these steps if you will operate the engines for a high power
engine run.
1) Make sure that the forward wheel chock is six to twelve
inches in front of the tires.
NOTE: This will cause the thrust of the engine to be held
by the frictional force between the airplane tires
and the ground, and not the wheel chock. The wheel
chocks do not have the same frictional force as the
tires. If the tires touch the wheel chock, some of
the frictional force between the tires and the
ground is lost, and the airplane can skid. The
wheel chocks are only used to prevent the airplane
from rolling if the airplane brakes were
accidentally released before or after the engine
run.