I am a little confused about what type of play you are describing. I do know that 'back in the day' when teaching lay ups, players were told they could take 1 and 1/2 steps, but I was never sure how I could take a 1/2 step, but knew what they meant.
A phrase that may help you better would be "you can pick your pivot foot up, but can't put it back down". You also can't pick your pivot foot up and than hop on the non pivot foot.
Hope that is a simplified answer for you. Now, if you want to get technical.....
