Section 20 adds an element of wishywashyness to BC's land title system.
In BC, a properly executed Form A (the transfer document) is effective against the current owner who executes it, even if not registered. In Alberta, it is not effective. Generally speaking this means that if you sign a Form A and then die, the Form A can still be registered.
So normally this crops up when someone executes a Form A, sticks it in a drawer, and dies, and then when it is time to divide up the estate, the Form A appears and to the shock of the grieving children or spouse, the family house suddenly doesn't end up in the estate.
In order to have a joint tenancy, you need the four unities, wind, water, air, and fire. Err... unity of interest , unity of title , unity of possession and unity of time.
If you break one of the unities, it becomes a tenancy in common. In alberta to break a unity you would need to register the Form A. In BC all you need to do is sign the Form A. Which can be done in secret and pulled out when you need it or burned if you don't.
Now what you would need it for, I don't know. Contemplating a divorce and want to break a joint tenancy with your spouse on the sly? Sneakily preventing a wills variation challenge by gifting the house before death?