It depends, I was taught to spear/fast cut stuff under 6" by bending my arm and cutting where the undercut would be first then walking the saw back to the topside, cutting wide open the whole time, quickly. A little while back I fast cut a big cedar marm, like about 15" by 40', because the top of tree was dead and I knew it would hang up and get caught with the undercut/backcut method. I cut the bottom and then matched the cut from the top, it worked awesome but my boss was not impressed at all. It is not an approved method, probably frowned on by some, no doubt.