Tacit casting is the ability to perform pymary without verbal commands. Two things are generally required for a wright to interface with the khert and the movable Aspects that sit within it - a Rited connection and knowledge of the language Old Tainish. Both of these things are acquired through years of training and eventual initiation. Tacit casters, however, are born with a unique Rited connection already in place, one that skirts the edges of their consciousness. This connection, unlike the one created on the chest of conventional wrights, leeches thought and allows the tacit caster to work spells internally.
In battle, one vulnerability most wrights face is their opponent's ability to hear the spell they are casting before it's manifested, and act preemptively. Being able to call forward spells silently however, tacit casters don't have this trouble and are therefore particularly fearsome in battle. It is an extremely rare but extremely useful ability.
Scholars aren't entirely sure what determines tacit casting. Since it sometimes runs in families there may be a genetic component to it. For public safety, most countries require children who display the ability to be turned over to the government so their connection can be sealed off until they are old enough to learn pymary properly.