When voltage is high on the right and low on the left current does not flow. It's an open.
|Schematic Symbol for the Diode|
But when when do not have grace on the left side predestination is blocked. The right side is cut off, and you cannot say that my condemnation was predestined to happen by God's will. It's no good to say that God demanded I be damned.
It's an imperfect illustration if you press it, I like that we have a physical illustration in nature of something that is clearly a one way street. God get's the credit for the good, and we get the blame for the bad.
J Vernon McGee used the illustration of a doorway of salvation; on the other side is written the words 'predestined' but you could not read that off the post until you pass through the frame. It's sort of the same illustration, you cannot blame God for your own faults, you cannot have the same perspective for both events. If we are damned it is because we have chosen it against His pleas, and if we are saved it's because He had mercy on us.