[HugoTafurst]

http://usssa1.com/FPUmpires/2012FPRules.pdf

RULE 6. PITCHING RULE
Sec 1.
A legal pitch shall conform to the following:
A. Prior to the pitch, the pitcher shall put both feet on the pitcher’s plate. Part of the pivot foot must be on top of the pitcher’s plate. The non-pivot foot must be in contact with the pitcher’s plate. Both feet must Both feet must be within the 24-inch length of the pitcher’s plate.

http://usssa1.com/FPUmpires/Fast%20P...ces%202012.pdf

Rules Differences

PITCHING POSITION, PRELIMINARY (FEET)
USSSA 6.1.A Pivot foot partially on top of the pitcher’s plate (rubber); non pivot in contact or behind. Within 24”

ASA 6.1.A.2 both feet in contact with pitcher’s plate within the 24 inch width of the pitcher’s plate

NFHS 6.1.1 pivot foot on or partially on top surface of the pitcher’s plate; non-pivot foot in contact with or behind pitcher’s plate, both
feet must be within the 24 inch width of the pitcher’s plate

NCAA 10.2.1 both feet in contact with portion of pitcher’s plate; pivot foot partially on top; within the 24 inch width of the pitcher’s plate


PITCHING POSITION, PRELIMINARY (NON PIVOT FOOT)
USSSA 6.1.E.2 One step forward; No step backward.
ASA 6.1.A.2 One forward step only
NFHS 6.1.1 Any step backward must start before the hands come together.
NCAA 10.4.2 A portion of the nonpivot foot must remain in contact with the pitcher’s plate at all times before beginning the forward step

I'll go with the rulebook.