Slip bags (you know, like Tyson?) I see people using or talking about every now and then. Its definitely an interesting concept. Basically is small bladder that is hung at about forehead level which swings from side to side. Who likes these things? I think they are the most useless bag in the entire punching bag family. If you stick around here long enough you will hear Grey saying A. how the bag is pretty ineffective at teaching proper slipping and B. because when you slip a punch you must slip the length of the punch with your body not just with your neck. Its kind of his 1-2 punch.
Anyway I think he is right. I also would like to add that the bag is nothing like a punch. There are no punches in boxing that come remotely close to being simulated by the movements of this bag. I used to have a training tool which I used to practice slipping. It was a stick. Literally, just a stick. Let me explain. The stick is used to represent either a jab or a right hand. To this extent it did very well. I would place it on a wall so that it was pointing at my face at around eye level. The stick was roughly the length of an opponent's arm. I would practice several counters on it using the stick to simulate an opponent's jab and imagining the rest of his body with my mind. This way I could practice slipping his jab, the full length of his jab, and following through with the actual counter punches after the slip using his arm (the stick) as a indicator to know how the rest of his body looks.
I could place the stick so that it was higher and pointing down towards my face to simulate a taller opponent jabbing down at me. I could place the stick lower to simulate a shorter opponent jabbing up at me. I could place the stick at around hip level and have it pointing up at my face to simulate a rising jab.
I think its a great tool to train on because it gives you the feeling of always having a jab in your face. Literally you become very comfortable with an opponent throwing his jab at your face because through practicing slipping on the stick jabs have practically become an extension of your face. Or at least thats how it feels. That sounds kind of like it hurts but its actually a good thing and will make you more comfortable when opponents throw jabs and right at you.