When a sadhaka chants with the tongue, the sound vibrations travel through the ears and are meant to reach their destination, the mind. If the mind is absent, the sound vibrations do not reach their destination, and ideal Japa does not occur.
It is the Beauty of the Lord making Himself accessible to everyone. Each one of us can have our very own Lord, and in this form, He can become our cherished property.
Our responsibility is to chant the prescribed 16 rounds properly. If there is no concentration, then completing 64 rounds or even 108 rounds has little value.