Angular acceleration refers to the rotational forces experienced by the head during an impact. When a helmeted head undergoes an angular acceleration, it means there is a change in the head's rotational velocity over time. During an impact, especially in situations involving oblique or rotational forces, the head can experience angular acceleration. This rotational movement can result in the brain twisting or shearing within the skull, potentially leading to traumatic brain injuries. Helmets designed to address angular acceleration aim to reduce the rotational forces transmitted to the head upon impact.