Saw this guy tendered his resignation, then boss tried to counter offer him

he said its either promotion or he will leave because he already got another job offer

question is this:

1- I wonder how he did it. Did he signed contract with the other co, then current co counter offered him. Ok, let say if he decided to stay with the promotion offer, how does he escape with the new co if he had already signed contract?

new co can sue him in lieu of 2 months pay? in accordance to their notice period?

2- Or he could have told the new co offered him, to wait, let him decide, then he decided to tender resignation. Risky game, because its a game of who chicken first, he or the current employer.

but he knows what he is doing because he is now handling a lot of big projects and the current boss don't want to take the risk of losing him.