[ Tris has a hold of the power as well, and will help Rin draw it in if.. drawing it in actually works. If it doesn't, then this probably won't work out. ]

That sounds like this might help. A little, at least. Your flames are tied into you a little more than this is tied into me.

[ At 'this' she shifts the fingers on the hand holds the volcanic heat. ]

You can try to imagine it like catching the fire with an end of a stick, or candle. Collect what is under your hand, and bring it in.