At a suitable working height
If you have a good working height that allows you access to the branches under the trunk, you can limb the downward face while you are performing the normal limbing process. Normally, it will be enough to limb the downward face after about two rounds of branches. Move your right foot a little bit and turn to face the trunk. Support the saw by resting your arms on your knees/thighs.
With the trunk directly on the ground
When you have limbed the whole trunk on the right, left and top, turn the whole trunk allowing you to access the branches underneath. If necessary, you can cut the trunk to suitable lengths before you turn it.