Describes someone who holds a cock/spire upright. The reason the word spotter is used is because, just like in benchpressing, the holding/spotting is done by a third party representative. Spirespotting is most common before the spicy bologna is inserted into the vagina (or rectum, for you cumtheca spirespotters out there) during intercourse. In pornography, the most common case of this is when, during a male-female-female scene, girl #1 is about to get her peach cobbler rived, so girl #2 grips the hedgehog and guides it into girl #1's pudendal crawlway.
The coati Ben Coates was robbing a liquor store when he felt something from below the waist. He recognized it as a hand. He thought to himself, "What kind of person would give my johnson such a firm skyward grip? Oh yeah, a spirespotter."