Yeah I never have understood that phrase. The man does what is the right thing to do by trusting God. What is imputed? The man did the right thing.
Let me refer to scripture by way of how I would try and explain it...
KJV Zechariah 3:1-5
1 And he shewed me Joshua the high priest standing before the angel of the LORD, and Satan standing at his right hand to resist him.
2 And the LORD said unto Satan, The LORD rebuke thee, O Satan; even the LORD that hath chosen Jerusalem rebuke thee: is not this a brand plucked out of the fire?
3 Now Joshua was clothed with filthy garments, and stood before the angel.
4 And he answered and spake unto those that stood before him, saying, Take away the filthy garments from him. And unto him he said, Behold, I have caused thine iniquity to pass from thee, and I will clothe thee with change of raiment.
5 And I said, Let them set a fair mitre upon his head. So they set a fair mitre upon his head, and clothed him with garments. And the angel of the LORD stood by.
Joshua is a sinner. His filthy garments are his character. Purely from a grace perspective, Joshua's garments are removed without any assistance on his part. His sinfulness is taken away, removed, and replaced with another's... Christ's sinlessness. Joshua himself has done nothing. The righteousness he is clothed with is not his own. Yet here he stands, without condemnation, clothed in Christ's character before the Judge of the universe as if he had never sinned. Imputed as a gift, without regard to his past life, reputation, character, habits and addictions.
We can now approach and enter into the presence of God only because of Christ's imputed righteousness. It is as if Christ Himself is standing before the Father. We can enter into his presence without being destroyed regardless of how far we have personally progressed in our walk with Him. We may yet harbor evil habits... Wicked thoughts... Even addictions... Yet because we are clothed with Christ's imputed righteousness we are accepted.