Namely, the elbows.
Yesterday was outdoor training. Did a bit of wing chun and an important point from Guro was introduced - how the elbows are key to generating punching power. The elbow is a small point, but a very important element of the punch.
At arm's-length distance, the elbow must rotate to allow maximum reach and deliver the hips behind the punch (think about the right cross). Inside of arm's-length distance, the elbows need to stay pointed downward and NOT ROTATE. The key strength for these punches comes from the triceps and back. Even uppercuts and body hooks are thrown with elbows down, not up, and both are close to the body rather than wide outside of shoulder-width.
Thus, when working on trapping hands/wing chun, it is at close distance - YOUR ELBOWS STAY DOWN and close to your body. Punches can be vertical or horizontal, but the straight line "piston" movement is very important.
This small point can be a big point when you master it.
See if it helps.