This is an excerpt from Complete Guide to Slowpitch Softball by Rainer Martens & Julie Martens.
You'll see many unique deliveries by pitchers as they add their own personal styles, but most deliveries consist of taking one step as the pitch is thrown or not taking a step forward as the pitch is released. Let's consider the advantages and disadvantages of each type of delivery.
The one-step pitching delivery is widely used because it's the way that we all learn to throw, whether overhand or underhand. The disadvantage of the one-step delivery is that pitchers are reducing the distance between themselves and the batter, which further reduces the time that they have to catch a ball hit to them. Some pitchers are able to take a slight step forward and then immediately take several steps back to get set up defensively, but not all are able to make this adjustment. The steps in executing this delivery are shown and explained in the series of photos in figure 5.2 and in Defense→Pitching→Delivery Methods on the DVD.
In the no-step delivery pitchers place one foot on the rubber or in the pitcher's box and the other foot comfortably to the side and slightly behind them (see figure 5.3 and Defense→Pitching→Delivery Methods on the DVD). Some pitchers use the foot opposite the throwing arm that they throw with as the lead foot and others use the same foot. The key here is being comfortable and well balanced in this stance so that the arm swing when delivering the pitch is well controlled. Another factor to consider is whether it is easier for you to back up starting with your right or left leg.
If you can achieve the same level of control with the no-step delivery that you can with the one-step delivery, it's the preferred method because you are able to retreat and set up faster, increasing your chances of fielding balls hit up the middle and reducing your chances of getting injured.