Perhaps the demonstration used in martial arts classes where you put your extended arm on someone else’s shoulder, elbow upward, and try to lock it in place while they pull downward on it; then you experience much more success by trying to point at the wall behind them?
But that’s longer and more awkward, both verbally and conceptually; and could be less widespread than trying to carry a full cup.
Perhaps the demonstration used in martial arts classes where you put your extended arm on someone else’s shoulder, elbow upward, and try to lock it in place while they pull downward on it; then you experience much more success by trying to point at the wall behind them?
But that’s longer and more awkward, both verbally and conceptually; and could be less widespread than trying to carry a full cup.