Whether stored or not stored, computed fields have a compute method. The annotation on the compute method specifies when the re-calculation should run, depending on if it's stored or not.

For example (stored):

@api.depends('name', 'surname')
def compute_display_name(self):
    # This will be called every time the name or surname field changes
    pass

display_name = fields.Char(compute=compute_display_name, store=True)

For example (not stored):

def compute_display_name(self):
    # This will be called every time the field is viewed
    pass

display_name = fields.Char(compute=compute_display_name, store=False)