The reason for making your back cut flush with the top cut in a Humboldt face or flush with the bottom cut in a conventional face when cutting saw logs is because it Ieavs a flusher end of a saw log for less triming. To save production time. This is typically only done when droping timber into a good clearing out in front of the intended direction of the fall or "lead" if you will. With no other standing merchantable, non merchantable, defective timber, or snags out in front of the fall. If the area of your lead is completely clear. There is no possiblity of the tree brushing into another standing tree thus possibly causing the falling tree to slide back off the stump twords the Faller. If your timber is falling into or twords other standing. The higher back cut creates a stop. In case of the tree brushing back.
In short back cuts are cut flush with face cuts when leading into big clearings because it's perfectly safe and saves the cutter production time from not having to "trim the butt" like you would if the butt has a safty step. Like I said when leading into other standing. A safty step should be used. Keep in mind!
Often a lot of the safty step can break off when using a swinging Dutchman and brushing into other standing timber!
Cut safe, stay sharp, and be aware!