It's random, but the chosen orbs to queue up are chosen from edges of the piles that you have, also if you hold shift, that is a wildcard orb and will match any

It's random, but the chosen orbs to queue up are chosen from edges of the piles that you have, also if you hold shift, that is a wildcard orb and will match anything. If you do it with a 0x80 it will purge a good bit of the board