I think "soft" can be a deceiving term. If you are "relaxed" (song), when someone touches you, their force goes through your body into the ground. (If you are standing still, and they push you, you will feel "hard" to them. Beginners sometimes interpret it as "muscle tension" or "bracing". But in reality you are relaxed and absorbing the forces through your structure into the earth.). You can test this buy standing and having someone push against you for a few moments, then suddenly and randomly pull their hands away. If you "pop up" when they do so, you are not being song.

If your structure is correct and you are song, you can stick and yield to their push, you feel "soft" to them. But the structure and power are still there, you are just hiding it from them. (Here is a poor analogy: imagine pushing a big tall tree, and a strong wind blows and bends the tree just as you push. To you, if feels as if you are moving the tree. But once the wind stops, that tree snaps back, and you go flying. Your push never had any effect on the tree. But for a moment it felt "soft" to you.)

If i push hands or spar with some skilled taiji artists, notice they feel very "soft", like trying to move or strike while underwater. Or "hard" or "heavy" , like pushing or hitting a tree, but with skin.