found this in a website:
Another of Rachmaninoff’s most famous compositions is his Third Piano Concerto. This long and difficult concerto is extremely popular within the piano community for its difficulty and profound beauty. Again, this piece is extremely difficult and is known for its requirement of “knuckle-busting” technique.
wtf…
i think dey is probably just using it as a figure of speech
i think so too
Some people have even called it Virtually Unplayable… 
da way to bust knucles is on the cheekbone of a slow mofo.
'nuff said.
I’m playin dis and mah knuckles r still in tact 

my friend and I challenged each other…he’s playing Prok 2 and I’m playing Rach 3 and we’re going to see who wins competitions next year
Hahha i vil pozzibly challenge da ROB on diz delicate izzue 