There are some complicated explanations of what a node is. This is probably an over simplification, and goes along the same line as denton explained.
Consider the barrel as vibrating up and down, with 12:00 at the top, 3:00 on the way down, but straight, 6:00 at full down, and 9:00 as straight but on the way up.
Next consider that fast bullets exit the muzzle sooner than slow ones. And, the barrel is really only stopped at the top and bottom of the vibration cycle.
Lets consider each one:
12:00 - If a fast bullet exits at say 11:00 (not all the way up), and a slow bullet exits at 12:00, then you get a compensating effect. But this may be an edge of the cliff node. If the slow bullet exits at 1:00 and the fast at 12:00. then you get the reverse effect. So there is an anti-node right after 12:00.
3:00 - The barrel is at maximum speed and fast bullets will always be higher than slow, and this is bad. A real anti-node spot.
6:00 - The fast bullets exiting at 5:00 will be higher than slow at 6:00, so an anti-node. Then you get a good node between 6:00 and 7:00, potentially as good as the 11:00 to 12:00 node.
9:00 - Barrel at max speed again, but fast bullets exit low, and slow ones higher. May be volatile, but at least it is in the right direction.
I think it is real. You can control which node by adjusting muzzle velocity. You can see it the most at longer distances. Here is the results of a ladder test I did at 300 meters last week. You can see where velocity goes up but POI elevation is flat or even goes down. Even with different bullets it can happen at the same velocity, because velocity determines when the bullet gets to the muzzle.
There is a lot of info out there on the method. But this is probably one of the better ones.
Incremental Load Development Is it necessary for a hunting gun? Probably not! It will not make a silk purse out of a sows ear.