Locking the two ends and moving the middle with sword
The video showed 3 moves of the Practical Method Straight Sword form, namely, Left cut wrist, Right cut wrist and Sweep down one thousand soldiers.
To execute either of the cut wrist moves, we must lock the tip of the sword in the same place in space and move the handle of the sword.
Considering the sword being an extension of the hand, the tip of the sword is one fixed end, the feet are the other fixed end. The middle in this case is the waist, and we use the waist through the hand to move the sword handle. This clearly illustrates the concept of locking the two ends and moving the middle.
By moving the middle, we send the energy to the tip of the sword, which represents a single lock that is only allowed to travel in a designated path. It is easier to illustrate this concept with a sword because a sword is a inanimate (dead) object. However, with empty hand, it is much harder as the hand is a live object and can easily move itself.
We must treat the hand as a dead object as well to make it a weapon, in particular, to forge a 7-inch knife (7 inches down the side of the palm from the tip of the pinky).