You do realize that shockwave that cuts mountains around for miles has unimaginably far greater force to generate it???What other explanation is needed? What makes you think the PS slash is a thing? Let me know. Over hyping every move just makes trolls more numerous.
The so called PS SLASH that made the shockwave had nothing to oppose it.
The one that didnt was met by something with equal or greater force. Example. Hashis mokuton , Kyuubi tail.
Its a regular slash. When anything is slashed at a certain speed unopposed, it produces a shockwave. Go ahead. Swing something really fast. It cuts the air producing a shockwave and distinctive sound. If something else is swung and collides with what you swing or catches it, shockwave nulled
You just cant compare it to whistling sound you have when you swing something.
Therefore the same force that cut mokuton would brake that mokuton all the same unless it wasnt the same force behind that swing.