At first, Kayin didn't have anything immediately in mind after Rarity put it like that. And while she was about to suggest they both finger and fondle each other... a thought at the back of her mind made her stop and focus.
Spoiler (click to show/hide):
figure first use, can be a pass, but future times if distracted or busy roll for the magick
Feeling a stirring of energy within her, she imagined a ghostly, ethereal tongue lapping at Rarity's folds and let the energy flow into reality, squirming as she pushed her hips back in an obvious display of need even as she hopped the idea and feeling would work.