The concept of "Ju" in Judo quite simply is "non-resistance". That if an opponent can put up resistance to your actions, you must be skilled in neutralizing that resistance. If you can neutralize any resistance your opponent puts up against your technique, defeating them regardless, then you have made your opponent "non-resistant". It's not that your opponent isn't resisting, it's just that due to the skill of "Ju", their resistance no longer matters.

For example, if your opponent pushes you and you push back, this creates resistance. If when your opponent pushes you, but you pull them while they are pushing, causing them to trip and fall, you have applied the principle of "Ju". Rather than fight their resistance, you neutralized their resistance. This is the idea behind related concepts such as "push when pulled, pull when pushed," that teach more specifies applications of the principle.

‍