I think there are two definitions being used here
There is a vibration node where the barrel the barrel doesn’t move but the area between that ‘node’ moves, and the area after it to the muzzle also whips
There are ONE of those every barrel every firings
The other ‘node’ is on the target
Starting low, increase velocity to the max
You should find a velocity where groups tighten
Increase above that groups open
Keep going faster, groups tighten up again
Keep going faster and groups open
Keep going faster and groups tighten
This will keep happening until you max out on pressure
Low node shooting is finding that first tight grouping at the lowest velocity