You walk up and it magically opens.

No knocking, Never knock

guys are prone to repeat the same old tired lines in reviews.  The door never magically opens.  Except when a K-girl named Magic happened to be working some years ago.

I don't know that the other replies explained it clearly yet, so here goes, based on my east coast experience.  

After the second call, you make your way to The Door, wherever it may be. As you approach the door, it magically opens, with no knocking, bell ringing or other request on your part and with no apparent noise or movement on the inside.  

After the call, it can take one to five minutes or more to get to The Door, but it always seems to open just as you arrive, as if by magic. I have waited for the elevator, gotten to the floor and - poof! - the door opens. I have gotten lost in the building complex, found my way to The Door and - shazam! - the door opens. I have gone to the WRONG door and - no surprise - it did not open! I realized my error, went to The Door and - abracadabra! - the door opens.  

One place I know well makes it clear on their web site: "do not make any noise (that might disturb the neighbors), do not ring the bell, do not knock on the door!"  You don't have to. As you approach The Door, it opens.

But it's after you're inside that the REAL magic takes place.  

K-girls aren't the only ones who can do the magic door trick. At many non-K indy in-calls, you can hear the peep hole sliding or maybe there's even some peek-a-boo as you hear and see her open the door. There is a certain consistency in door technique, but especially at the KMPs and apartments.  

repeat after me, "There's no such thing as magic."

Kudo's to the guy who invented peep-holes.