The catcher will have a specific touch signs to inform the pitcher what type of pitch to throw.
For example, the catcher may touch his chest (a key) to indicate the next touch is real, then give the sign.
Typical touch signals catchers could use include:
Fastball: First touch of face mask after key
Slider: First touch of right ear after key
Change Up: First touch of chin after key
Second Set of Signs: Pounding twice in mitt and look for 2nd touch
Hope this helps.