https://lparchive.org/Fatestay-night/Update 48/
"Can you see it, Saber?
Swallows avoid the sword by feeling the wind. It does not matter how fast your sword is. You cannot swing a sword without shaking the air, correct? The swallows feel this shaking and change their direction of flight.
So no one could kill the swallows with one blow, no matter what kind of blow it was. A sword is only a straight line after all. It is only natural for it to be unable to apprehend the swallow, which can move vertically and horizontally."
Thus you only have to surround its escape route.
Use one blow to attack the swallow and use your second blow to catch the escaping swallow as it reads the wind.
But they are fast. The second blow cannot come in time with this longsword. If I wanted to get the swallows, I would have to do both attacks at about the same time. But such a thing is beyond human abilities.
I accepted such a thing was impossible, but
Unfortunately, I had nothing else to do.
They say determination is key to success, but when I noticed, it was like this.
A worthless idea to kill a swallow became a secret technique that creates a prison with many slashes.
In her mind, Saber disagrees with Assassin's words.
It's wrong.
That technique is not as simple as that
About the same time? No.
The two blows came at exactly the same time.
Assassin's Sasaki Kojirou's longsword doubled it's existence for just that instant.
"Multi-dimensional refraction phenomenon… Kishua Zelretch… A Servant that has reached the level of a Noble Phantasm with mere skill"