Compressive stress from the bottom

1. Start with the undercut up to about 1/3rd of the trunk diameter, or until the guide bar tends to pinch.
2. Now cut from the top to meet the first cut.

If the trunk is thicker than the guide bar length

1. Start by crosscutting the opposite side of the trunk.
2. Pull the saw towards you and cut a little from the top.
3. Now crosscut from the underside, up to about 1/3rd of the trunk diameter. Make a bore if the trunk is lying on the ground.
4. Finish with a top cut.