In his book More Mysterious Wales, Chris Barber writes:
Standing in the churchyard [of St. Mary’s Church, Welshpool], this hunk of stone is reputed to have stood formerly in the abbey of Strata Marcella (SJ25131042) where the abbots were ‘installed’ on it as part of a well established ritual.
After the Dissolution of the monasteries Maen Llog was moved to St. Mary’s Church, and a new ritual grew up around it:
Folk who were required to do penance were made to stand on the stone, dressed in a white sheet, with a candle in one hand.
The stone was moved into the graveyard on the instruction of a Puritan called Vovasour Powell, who considered it to be an object of superstition. It sounds like Powell was right, because the stone came to be used as a wishing stone. To make wish, you have to climb it and turn three times to face the sun.