You lean forward into not away from brambles like a normal person, shinny your unprotected forearm into narrow thorn-thronged passages when blackberrying. It is a state of mind as much as it is an activity of the body.
You must forget the goal, and simultaneously have only the goal in mind. It's like Zen archery. You must intuit the nothingness that is the fat ripe delicious warm globe. You must know that already you and the blackberry are one in this phenomenal universe.