Can some explain witches sabbath to me?

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The description says "Summon up to 3 highest-power units from your opponent's graveyard to an enemy row, then Summon as many highest-power Monster units from your graveyard to the opposite row and give them Doomed."
But every time I used this card it chose the strongest units by neither of us, going even as far down as summoning the 6. highest unit! How does this exactly work? is the same amount of revived cold gards needed or does it just choose random and lets you loose a match if you have bad luck or is there some other hidden mechanic?
 
Mate, it's super simple: the script starts picking from the highest power unit to the lowest. If for example there are multiple cards with a power of 6 in your grave and it is currently the highest power card there, the Sabbath will pick one at random.

Just look into your grave, identify 3 cards with the highest power - these are the ones that could be pulled by Sabbath.
 
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