This starts to train the basic sensitivity and also teaches ward off, roll back, press and push.
As you are only using one hand then you can concentrate more on correct technique and relaxing into the movements.
Stand opposite your partner both hands touching at the wrists. Your partner pushes gently towards the middle of your chest, you ward off by gently moving the arm outwards then roll back to draw your opponent forward. They feel when the push is complete and you gently press down, they relax and you initiate the push and the exercise continues.
It is important to keep both feet relaxed but rooted and do not use speed or force, but try to feel the opponents weakness in root and body weight.
Sun Style push hands is done relaxed and slowly as it is non competitive, but working with different partners to adjust to varying strengths and weaknesses. The training is mutually beneficial but ultimately to know yourself.