Dallben would consult Hen Wen, the oracular white pig, through the use of the sticks, but only in a matter of greatest urgency. The great enchanter attempted to do so during the rise of the Horned King, but Hen escaped before Dallben could retrieve the rods; and once, successfully, later after Dyrnwyn had been stolen by Arawn.
Instances of Use
The first time Dallben wished to use them, within Taran's seeing, was during the events of The Book of Three, but the enchanter could not find the rods in time. The only instance Taran ever saw of the Letter Sticks was at the outset of the events of The High King. In the latter episode, the rods exploded before Hen Wen could complete the final prophecy.