>>40511067Official material for XY on the website for it (taken down now, replaced by the general page for XY on the main site) described Mega Gardevoir, and stated the horn was a physical manifestation of the its heart, and that it increased its power by opening its heart (in a literal sense, since the horn splits in two).
I spent the last like 15 minutes googling for a screenshot but couldn't find one, nor can I get to the page in Wayback machine. So I guess you'll just have to trust me and Bulbapedia and a bunch of sites from late 2013/2014 quoting the line
>"The red plate in Mega Gardevoir's chest is said to be a physical manifestation of its heart. Mega Evolving enhances Gardevoir's Sp. Atk significantly.">"By opening its heart, Mega Gardevoir becomes able to use tremendous psychic power" 