Those are bad pieces of machinery right there! You can set that cutter head up a couple different ways. One is manual to cut-to-length when you pull the trigger so to speak. Another is to measure and cut at an exact pre-set length (like 12'4" or 16'4") Some of those heads are computer run to scan the size of the log as it goes up limbing it and cut at whatever length will provide the most board footage of lumber (it actually reasons that there is one 2x10, 2-2x6es, and 3 8foot 2x4s if it cuts here as opposed to somewhere else). All computer driven. The operator just puts it on the tree and it decides the rest like that. It's very popular in the northwest but still rare in the southeast were I'm at.