well, its not easy to spot those hotlinks or call back since they use it to protect their product. So you have to understand very well PHP, and go thru the code, and understand what they write, then you might spot them ( tthose hotlink)
I've seen how GYSN does nulling, first releasing a keygen version and after a couple of days full nulled.
There's a much simpler way, and the longest process would be removing your license number and time of download.
I'm not gonna give away my secrets, since I wanna be the first who Nulls and Releases vB 3.7 B2 today. But the process is to just remove the process where your server checks for the license.