Its just experience Shadye.
+60 years worth.
That plus the fact that I've handled literally thousands of blades and have been taught by probably the best and most knowledgeable teachers of their generation, two of whom were Surakarta Karaton Empus.
There is a good Tuban blade shown in this thread, and Jean has shown a pic of one of the super-size Tuban blades:-
http://www.vikingsword.com/vb/showthread.php?t=22862