Have you taken up boxing recently and perhaps finding that your neck is stiff and sore afterward? Chances are you are not holding the boxing mitts/ focus pads in the correct manner. When your partner is punching with you, they are aiming to hit the boxing mitts hard and in the centre. For you to protect your own body from the power of the punch you must position yourself properly.
The first thing to do is to ensure that your boxing partner is wearing correct boxing gloves and that they have their boxing technique correct. If they let fly with a punch you are not expecting, then it can really damage your own body.
Next thing is to ensure that you are standing with your boxing mitts up and in front of your body. If you have them out to the side of your body then all the force of the punch gets taken in your arm and shoulder. If you have the boxing mitts in front with your elbows down beside you then the force gets taken by your body, not your shoulder. This helps to diminish the risk of shoulder or arm injuries.
The next very important thing is to ensure that your shoulders are down and relaxed. This stops the force from the punch travelling up into your neck. Now when you are taking the punch into the boxing mitt it is vital that you give with the punch. This absorbs the force somewhat, diminishing the chance of you getting injured. Once you have got this technique correct and you are using superior quality boxing mitts, you should be able to box with a partner with no damage to yourself!