It is forbidden to speak, motion with head or finger movements, or do any type of work, even if it doesn’t require any concentration, while  reciting a brachah. For example, while saying Birchas Hamazon one should not gather the crumbs on the table. Similarly, this is the reason why on Succos when shaking the lulav during hallel one does not shake while saying the name of Hashem. B’dieved if one did do something while reciting the bracha one does not need to repeat it.