Tense your arm and shoulder at the moment of impact when jabbing (unless you are throwing light, distracting feints). You will find that doing this will often cause one or both pecs (chest muscles) to tense, as well as muscles in your neck and back, such as your trapezius, your punching-side lateralis, and the smaller muscles supporting your scapula.
When punching whilst moving and when punching for reasons other than maximum power much of your body can remain loose and mobile, but when throwing a very hard punch such as a well-planted straight right or a stiff jab, you may actually tense your whole body momentarily upon contact, optimising the impact of your strike.
Note though that this tensing of the muscles will ideally become an unconscious process. Next time you hit a heavy bag, you may feel your body automatically bracing for the impact. If you don't do this, your punches will likely feel weak and you will may feel as though you are losing balance and/or being forced backwards. You will also find that no matter how well you brace at the moment of impact, your will lose power and balance if your posture is incorrect, and small improvements here (together with range and timing) can lead to substantial gains in your punching power and efficiency.