I don't think I would ever attempt this on a free-standing log as pictured below, a fallen tree might be do-able ...
Buck a Log With an Ax on Field & Stream:
You round a corner and groan: A fallen tree blocks the road. Your buddy’s default response is to stomp to the back of the truck, pull out the chain saw, check the chain tension, futz around for premixed fuel, try to find earplugs, and look for safety glasses. By the time he walks up with chain saw ready, you’ll be well on your way to a bucked tree and a clear road with little more than an ax and attitude. Smirking follows.
Chop, Chop Stand on top of the log and chop a V-notch into the side of the log between your feet, using a six-stroke count: Make three swings angling in from the right—the first one high on the log, then low, and then in the middle. Next, repeat with swings angling in from the left—high, low, and middle. On that sixth and final stroke, flick your wrist slightly outward—an inch will do it—right after the bit bites wood. This will help toss the chips out of the notch and prevent the ax from sticking. Cut halfway through one side of the log, then turn around and chop another V-notch through the other side. Plan the Vs so the tips of the two notches are slightly offset. This prevents the final stroke from overtraveling, sending the ax bit between your legs and that strong hickory handle into your nuts at warp 9.