You can interrupt a combo at just about any time by throwing a shiruken. Thus the term "shiruken cancel".
You can also string together combos by putting a shiruken cancel in and thus make "freeform" combos. I don't know any off hand but one might be X, Y, X, X, B, OL Y, B, X, Y, X, X.....etc.
This is a technique that is very useful and has been in just about all the current Ninja Gaiden games (Ninja Gaiden, Ninja Gaiden Black, Ninja Gaiden 2, Ninja Gaiden 3). I don't know if they had it in the NES games.