Coin adding has to do with adding a coin to a combo after having already started the combo by shooting a different coin. For example, you throw a coin behind a Cerberus, shoot that coin through the Cerberus with your electric railcannon, and then quickly switch back to your pistol and throw another coin. The railcannon hits your original coin, and then reflects back and hit the second coin as you are throwing it. That would be a coin add.
If you haven't seen it before, I strongly strongly recommend watching HerbMessiah's video on coin mechanics, it's very good. If you aren't willing to watch the whole thing, 14:05 is the timestamp if you want to watch specifically the part on coin adding.
Its really fun is what it is, but the vid does explain every thing really well
If you have not seen ULTRAKILL is finally having a speedrunner competition for real money!!! It starts in about four hours from the time I am making this news post, in fact its probably already started when you are reading this so click the challenges widget on the right and read up on the rules and
