In weapons work it is my understanding that one does not always stay physically stuck to their opponent’s weapon. There are practice drills where remaining physically in contact is important (and it is important to have this sensitivity/ability), but in free fighting there is often disengagement from the opponent’s weapon. The response to “beng” is an example of this.
In my understanding, defense against “beng” is dependent on having the proper grip, that is, not so tight that when your weapon receives force it is transmitted to your arm (and through that to your structure), but not so loose that your sword gets knocked out of your hand (or if not lost entirely, then at least enough to reduce your control, prohibit you from protecting the centerline…). If you have the proper grip, then, with a little sensitivity and guidance, the sword (if properly balanced) will almost on its own swing to the opposite side of their sword. Most attacks of this type, unless very skillfully applied, use a longer energy than desirable leaving the attacker unable to guard properly from attacks initiated from this new side that you are now on. Part of the sword movement to the other side is dependent on a properly balanced sword. The point of balance (POB) on historical weapons is about 6-8 inches beyond where the handle meets the hand guard. This POB then becomes a pivot point for the response. Note that if the POB is too far out then the sword will not be lively, and if too close in towards the guard (as is somewhat common in reproductions) the movement of the sword will be primarily in the practitioner’s wrist preventing the sword from being controlled properly. You can think about the root of the sword (near the grip on the handle) as if it were analogous to the shoulder when without a weapon, the POB as analogous to the elbow, and the tip as analogous to the hand/wrist.
For those without sword experience, you can experience a similar effect if you have a partner suddenly try to slap your arm to one side or the other. If too firm, your body may be jerked to the side, but if too lose, then your arm may fly away out of control. Just the right amount of resiliency in the arm, however, and you can easily snake your arm around to the opposite side of theirs. If you can do this, note the movement around the elbow as this should be similar to how the sword should move in response to the “beng” energy.