You keep your tongue to your self to give your heart a rest. You’ve been talking and moving your tongue around all day expressing your feelings. This has tired both your tongue and your heart out, so you stop your tongue moving and decide that you need some self rest time.
What better way to rest than to eat some cake (けい)! No talking necessary. Just enjoy the delicious taste of cake on your tongue and feel your heart let out a sigh of relief.