I think you're talking about "pin gap shooting" If you have a 20, 25, 30 yard pins and the deer is at 22.5 yard then you put the 20 yard pin above and the 25 yard below.

One pin shooting is really having only one pin. Most hunters sight it in at 20 or 25 yards. When practicing, they learn how high or low to aim depending how close or far the target is from 20 or 25 yards. this method is very good for faster set-ups. For slower set-up multiple pins are usually better. Example - 10, 20, 30 to your max.

Hope tis helps....Good Luck!