Actually, there is a very specific technique that I was taught long ago in secondary agricultural school, and then in practical classes at university; first aid kits were mandatory in classes :).
Very rarely can you see this very technique on youtube. At that time I didn’t understand how the cut was done either; but once it “clicks” you can cut the thinnest scions with a straight cut indefinitely.
The thumb is always placed exactly opposite the blade, but it is stiff and presses straight against the blade but does not move along the blade!! the blade should slide along its (almost) entire length through the scion, and not be pushed with one part of it through the scion tissue; the hand holding the scion is fixed to the body (chest), and the hand with the knife pulls laterally (as if you are going to elbow someone).
