Best description of IT I've seen is from On The Road by Jack Kerouac:

"Now, man, that alto man last night had IT-he held it once he found it; I've never seen a guy who could hold so long."

I wanted to know what "IT" meant.

"Ah well"-Dean laughed-"now you're asking me impon-de-rables-ahem! Here's a guy and everybody's there, right? Up to him to put down what's on everybody's mind. He starts the first chorus, then lines up his ideas, people, yeah, yeah, but get it, and then he rises to his fate and has to blow equal to it. All of a sudden somewhere in the middle of the chorus he *gets it*-everybody looks up and knows; they listen; he picks it up and carries. Time stops. He's filling empty space with the substance of our lives, confessions of his bellybottom strain, remembrance of ideas, rehashes of old blowing. He has to blow across bridges and come back and do it with such infinite feeling soul-exploratory for the tune of the moment that everybody knows it's not the tune that counts but IT-" Dean could go no further; he was sweating telling about it.