Playing the neutraliser role is one of the four roles that muscles can perform, the other being agonist, antagonist and stabaliser.
In many body movements co-ordinated contractions of muscles are required. When this is done one set of muscles is said to neutralise part of the functions of another set.
So when a muscle acts in the role of neutraliser it causes the opposite movement to that of another muscle. Both muscles contract, and each equalizes or nullifies the effect of the other muscle's contractions.
E.g. To perform a sit-up the right and left external obliques both contract, cancelling each other out, resulting in the flexion of the trunk forwards.