In Yoshi's Story, the Smile Meter's first appearance, the Smile Meter appears in the upper-left part of the screen.
When a Yoshi touches an enemy or eats certain things (such as Peppers, Bomberries, Dirty water, Ick Worms and Black Shy Guys), the petals will gradually fall off. When the Smile Meter has no more petals, the Yoshi loses a life. When falling in an abyss/lava getting swallowed by blurps, getting crushed by moving gears or getting crushed by a compressor, all petals are lost. Even as Super Happy Yoshi falling into an abyss/lava getting consumed by blurps/getting crunched by moving gears or compressors still takes all the petals off the smile meter. When a Yoshi eats fruit, petals can be re-gained.
Yoshi's Island DS
In Yoshi's Island DS, Yoshi can lose health while trying to hold on to one of the babies. The Smile Meter is the same as it works in Yoshi's Story.
Super Mario 64 DS
The Smile Meter is also in a Super Mario 64 DS minigame, "Loves Me?", where the player touches the petals to make them fall off. At the end of the game, the flower will then decide.