First of all, there is more to that knife defense issue than you could ever get from people in the internet, so you will at best get a small idea of what you could do or how you could train.
I've posted something about knife confrontation in another thread
So, concerning your premises (I hope I've got a correct picture of the situation in my mind as I'm not a native speaker): If you can't run (which should be your primary approach in all weapon encounters) do as the others already have told - give him what he wants. Neither your wallet nor your iPod is worth your life! Most people underestimate how dangerous a knife can be and what it can do to them. They usually know at worst a cut in their finger with a kitchen knife which slipped while preparing their meal, but this is nothing compared to an intentional knife attack. An acquaintance of a very good friend of mine got stabbed not long ago with a knife and was in coma for more than a month, but at least was lucky enough to survive (the attacker missed his heart by half an inch). In the following videos you can get an idea of what might happen to your body if someone attacks you with a knife.
So again, if you can't run away, give him your money, mobile, iPad what ever IF it looks like that that's what he is after. There is a pretty good chance that he won't harm you if he didn't attack you in the first place, but only threatened you to give him your stuff. Of course that will never be a 100% chance and you need some good empathy to judge how he will react after you've given him what he wanted.
IF however you have the strong feeling that he's seriously going to harm you even if you do what he asks - then you must defend yourself at full force. Grab his knife arm with BOTH of your hands. I say at forearm at elbow level, because that part he can't move as fast as his hands, so it will be harder for him to avoid to get grabbed. Going for his wrist however will give you more control over where the knife actually is. Try out in training what is most comfortable for you. There are people who don't recommend to go with both your hands, using the argument that he would attack you with your free hand, but arguments and also my own experience strongly advocates using both your hands on the attackers knife arm.
- The knife is the biggest threat in that moment! Using only one hand on the knife arm and your second hand on his other arm will not only put a smaller force against the biggest threat, but most likely have you also using your strong hand (right) against his weak hand (left) and your weak hand (left) against hist strong hand (right)
which makes it easier for him to keep his knife free and attack you with it.
- Someone who is not trained in knife fighting will usually be absolutely focused only on his knife. That means that he probably won't really attack you with his free hand, but rather go with it for your grabbing arms to get his knife free. You can try that out in training.
My advises here are for a training against attackers who are not highly trained in knife fighting. Against those who are, there is not much you can do in an encounter and even less you could learn from the internet. To raise your chances from zero to minimal, you will have to train with people specialized on knife fighting.
So when you have control over his knife arm you want to stomp kick on his leg, preferable his knee and inflict as much damage as you can so he won't be able to follow you when you run away if you have the possibility.
Jon Fitch does pretty well in controlling the knife arm of his opponent in this clip:
You may also try to get a copy of Jim Wagner's reality based personal protection knife defense DVD. A bigger part in the end of it is advertisement for his knife, but the training instructions part before that is pretty good.
If you can afford one, get a shock knife like in the Fitch video for your training. It lets you know every mistake you do while you practice. But you should also work with a color marker knife (a marker pen does also) and a white shirt. With that you can evaluate you training afterwards. And ask your trainer to invite some acknowledged knife fighting instructors to give seminars at your gym.
Always keep in mind that in a knife attack encounter there is NO safe way. No matter what you do, there is ALWAYS the chance that you get hurt. All your training and actions are about minimizing the risks for you and to get as little harmed out of the situation as possible. So, to sum it up:
- First priority is to run away
- If that's not possible, give your money, mobile etc.
- If the attacker is going to attack you no matter what you do, control his knife arm and inflict as much damage as you can and run away at the best opportunity