Mages can cast spells in a single turn without making any outward sign.
A mage in a grapple (whether he initiated it or not) can cast a spell whether he wins the grapple check for the turn or not, but suffers a –3 penalty to the spellcasting roll if he lost.
Mages in a grapple cannot employ any Yantras involving movement, but if the grapple was initiated part-way through casting a spell, any Yantras the caster has already spent turns incorporating into the spell have their full effect.