This is how the rule works:

If player gets a Blackjack composed of an Ace and a J, both of spades, he gets paid INMEDIATELY (before dealer takes any action and regardless of the upcard) 4 times his bet.

The blackjack remains in play, which means he could then be paid the normal 3:2 payout or tie if dealer also gets a blackjack.

2 questions regarding this situation

1) What would be the effect on the house edge? Game is 6 decks.

2) Considering it is a low chance occurrence, would you still add the result of question 1 into your edge for your betting ramp in a counting game?

Thanks in advance