Hello, cracking a line is simply loosening the nut that clamps the line to the top of the injector. Don't take it all the way off, just loosen it enough to release the fuel pressure. small amount of fuel will bubble out of the top and bottom of the nut each time the pump delivers to that injector.

Nailing.....I have heard of it as well but I am not sure what it is.

Cheers,

EddieE
87 300SDL 325K