Why would anyone exert Trueheart Twins? It doesn't have an exert perk.
You may exert Trueheart Twins as it attacks. (It won't untap during your next untap step.)
Whenever you exert a creature, creatures you control get +1/+0 until end of turn.
To contrast it, here's Nef-Crop Entangler. Notice how this card says "when you do [exert], it gets..."
You may exert Nef-Crop Entangler as it attacks. When you do, it gets +1/+2 until end of turn. (An exerted creature won't untap during your next untap step.)
Here's a list of creatures with exert. Nearly all of them follow the latter format. (There are a couple, but not very many, that seem to have no exert perk)