A "twisty" shank is the result of an aggressive climbing plant (like honeysuckle) pressing its attentions on a young, growing stem of a tree. Here it's latched onto a beech sapling. In this case, the stem is already a bit on the thick side for stick making and the creeper hasn't managed to make much impression on it so far.
(Wyre Forest, Worcestershire)
This one didn't fare so well. You can see that it's had quite a battle with a creeper that has cut deep into the wood, which responded by growing around the intruder. This is how a good twisty shank is formed, though this one is much too thick for our purposes.
(Wyre Forest)
Here's a small hazel stool infested with the stuff - but again the creeper hasn't bitten into the wood yet and the stems will be too thick by the time it's had any significant effect.
This is why good twisty shanks are difficult to come by. To be truly effective, the creeper must grab a slim, fleshy young stem really aggressively, forcing the stem to grow around it, but not killing it in the process. It takes two to tango, and as you can see, this doesn't happen often enough for we stickmakers!
P.S. No, I wasn't cutting shanks in the Wyre Forest, just admiring the scenery!
(Near Wareham, Dorset)