Muscular contraction means a loss of speed, flexibility, and you telegraph your intentions. Soft hands, in contrast, are deceptive, their hearing is far more acute, and they can rapidly respond to changing circumstances with ghostly proficiency. With a good player, you hardly feel his defense or attack. All you feel is the impact of his blow and you're not sure how he did it. He uses what you give him and he's not out of breath doing it.
Soft arms conserve energy.
It's not logical to expend great amounts of energy with rigid defense movements leaving little in reserve for attacking, assuming you get the chance. Soft arms don't give information either. Keep your elbows low along the centerline and continually remind yourself that they are the source of forward energy. Eventually, they'll get the message. Likewise, keep the wrists soft and pliable, allowing them to merely stick on your partner's.