The Patriarchs–Abraham, Isaac, and Jacob-reached such spiritual heights that we cannot distinguish them from the heavenly chariot that Ezekiel described in his prophetic vision.
The rider stands on a chariot and all can see him.
The chariot is a vehicle that has no independent status.
It neither determines its direction nor the particular road on which it will travel.
It is just a vehicle that brings its rider to his destination.
When a person is a chariot of holiness, his entire being expresses holiness, the divine Being within the world.
Whenever anyone acts in a holy fashion, he becomes a vehicle for holiness.
Still, that does not make him a "chariot."
However, the Patriarchs were different.
For throughout their lives they never for a moment ceased from binding their mind and soul to the lord of the universe, with the aforementioned absolute surrender to His blessed Unity.
When a person nullifies himself–even if imperfectly–for a sanctified cause, he becomes an instrument of holiness.
However, he is called a chariot only if he can live that way, only if his entire being, whether awake or asleep, conscious or unconscious, remains an instrument for the Shekhinah, for God's presence.
Some people become a vehicle for the Shekhinah by engaging in conscious effort.
But when that passes, the connection is broken.
However, for those rare individuals who become the chariot, the connection with the divine does not depend on conscious decision making.
Whether or not they intend them to be, their actions stem from the divine power at the root of their being, and they no longer act independently.
–Rabbi Adin Steinsaltz
From Understanding the Tanya, p.164, by Rabbi Adin Steinsaltz