Here is the cards in the combo:
Unnatural Selection:
1U Enchantment
1: Choose a creature type other than
wall. Target creature's creature type becomes that type until end of
turn.
The flagbearer ability is:
If a spell or ability an opponent controls could
target a Flagbearer in play, that player chooses at least one Flagbearer as a
target.
The idea is to change a creature's type to
flagbearer so that any spell that targets one of your creatures (like
Shock) is transferred to one of your opponent's creatures.
There is a slight problem: The Flagbearer's ability
that makes you target a flagbearer isn't actually part of the creature
type. So if you change a creature's type to flabearer, and no other "real
flagbearers" (like Standard Bearer or Coalition Honor Guard) are in
play, then the player may target anything he or she chooses. This is
also on the Apocalypse FAQ at the Wizards website at http://www.wizards.com/magic/expert/apocalypse/ap_faq.asp
-Ronald C.