Jonsered CC 2152C Operator's Manual page 26

Next make the bottom cut so that it finishes exactly at the
end of the top cut.
The directional cuts should run 1/4 of the diameter
through the trunk and the angle between the top cut and
bottom cut should be 45°.
The line where the two cuts meet is called the directional
cut line. This line should be perfectly horizontal and at
right angles (90°) to the chosen felling direction.
Felling cut
The felling cut is made from the opposite side of the tree
and it must be perfectly horizontal. Stand on the left side
of the tree and cut on the pull stroke.
Make the felling cut about 3-5 cm (1.5-2 inches) above the
bottom directional cut.
26 – English
WORKING TECHNIQUES
Set the spike bumper (if one is fitted) just behind the
felling hinge. Use full throttle and advance the chain/bar
slowly into the tree. Make sure the tree does not start to
move in the opposite direction to your intended felling
direction. Drive a wedge or breaking bar into the cut as
soon as it is deep enough.
Finish the felling cut parallel with the directional cut line so
that the distance between them is at least 1/10 of the
trunk diameter. The uncut section of the trunk is called the
felling hinge.
The felling hinge controls the direction that the tree falls in.
All control over the felling direction is lost if the felling
hinge is too narrow or non-existent, or if the directional
cuts and felling cut are badly placed.