Rule 805 Testimony in the form of an opinion that is otherwise admissible is not objectionable because it embraces the ultimate issue to be decided by the trie…

A witness can give their opinion about the most important question in a case, even if that question is the exact thing the judge or jury needs to decide.

For example, a doctor can testify that in their opinion a patient was not mentally capable of signing a contract, even though that is the key question the jury must answer. The opinion does not have to stop short of the final decision just to avoid stepping on the jury's toes.

The opinion still has to meet all other rules about evidence and expert testimony to be allowed in court. This rule simply removes one specific roadblock that used to prevent witnesses from speaking directly to the heart of a case.