What is Peng Jin?

What is Peng Jin and is it better to maintain a little in the arms for example to prevent people from coming in?

People misunderstand Peng. There is another word with the same sound and only one stroke different that means something like structure or framework and people often think this is what is meant by Peng. If you base your Taiji on this incorrect meaning of Peng then the whole of your Taiji will be incorrect. Peng Jin is over the whole body and it is used to measure the strength and direction of the partners force. But it is incorrect to offer any resistance. It should be so light that the weight of a feather will make it move. It can be described like water which will, with no intention of its own, support equally the weight of a floating leaf or the weight of a floating ship. Then he added in English: “Peng Jin is sensitivity”.

Reference:
Q & A’s with Master Ma Yueliang. Interview by Patrick A Kelly patrickkellytaiji.com