We use the sign for slow when we are trying to teach our babies to control their speed when eating, walking, or playing. This sign is best introduced by example and contrast. You can walk slow and fast or play a drum slow and fast to allow your baby to discern the difference.
Use the slow flash card in settings such as meals, to encourage your baby to slow down when eating. It is also useful in the playground, or as your child grows older, it becomes helpful in public situations like church.