Before the Edge Priory became the reclusive group they are now, they would channel their energy into steel lockets that could be worn by their Awoken Hollow protectors, allowing the Hollows to gain more strength at the cost of their longevity.
Channelheart's use clearly depends on how many Sting spells you have over the course of your run (see this for a list of ways to have more Stings), giving you +20 damage for each. Since Sting spells tend to hit the front enemy unit, which has enough to tank even boosted Sting spells except perhaps in Rings 1-3, this Artifact does not often change the complexion of a run that much, though its damage adds up provided you have enough Stings.
Be careful not to over-flood your deck with Sting spells even with this Artifact, as while Sting is "free" up to a point thanks to its +1 card draw next turn effect and 0 cost, if your deck has too many of them, you may not be able to take advantage of all the draw thanks to the 10 card hand limit, and even with the +20 Magic Power boost, you do not want Sting to replace other cards lategame.
See: Version History