From 6-1-1
He shall take his sign from the catcher with his pivot foot in contact with the pitcher's plate.

From 6-2-5
ART. 5... It is also a balk if a runner or runners are on base and the pitcher, while he is not touching the pitcher's plate, makes any movement naturally associated with his pitch, or he places his feet on or astride the pitcher's plate

Since taking the signal must be from the pitchers plate, to not do so constitutes an action naturally associated with the pitch without the pivot foot on the rubber. How do you catch it - sometimes just a nod of the head or shaking the head side to side to change a sign. A lot of guys don't call it for any number of reasons - it's a technical violation, etc. Some won't call it because they don't know the rule. If you call it a a frosh game you'll probably get an earful from the coach - "this isn't varsity, you know" If you don't call it maybe the other team will chirp in and say - "hey, isn't that a balk?"