Can use pain lands to trigger essence channler. You are unable to do this in declare blockers step. Must do it post attackers step. If you do not put a stopper up, the game will go straight to declare blockers, giving no chance to respond prior. I have had to forfeit many a game due to not having full control active for this. Unsure if it's a good idea to put some code where when essence and a pain lands are out, you have a response active, that would be nice.