Our explaination is likely quite different from most/many "Isshin camps"
The previous movements (what you call "double shuto") has an upward and downward aspect embedded in that movement. If successful with the upward portion the opponent will have their ribs in the ideal position for that short distance open handed palm strike.
If unsuccessful, (or merely not as effective as planned) the downward aspect can redirect their arms-body just enough to slip the palm inside the ~opened drawbridge~ of their floating ribs.
Something to consider, and as always I could surely be mistaken,
Jeff (Gladly can explain in smaller detail if not sufficently clear re: the so called "double-shuto")