Someone posted a Dan Inosanto video in the JKD forum that had some great examples of the interchangability of Kali. Guro demonstrates how, for example, an elbow strike or hammer fist can easily take the place of a strike with the knife. I am also constantly reminded by my own Guro that when fighting with a knife, it is best to still think of the hand that holds it as a fist, otherwise your concentration is too distracted by your weapon and you forget about your other hand and legs a little. The thing I love about Kali is that much of it can be figured out just by experimenting. IE, do some knife drills, drop the knife and ask yourself "hmm, how would this work unarmed?" It makes for some incredible defenses and counter attacks.