A more thorough explanation:

The attorney-witness rule, also known as the lawyer-witness rule, is a principle that states that an attorney who is likely to be called as a fact witness in a trial cannot participate as an advocate in the case. This is unless the testimony is about an uncontested matter, the amount of attorney's fees in the case, or if disqualifying the attorney would create a substantial hardship for the client.

The rule allows an attorney who is actively participating in the case to be a witness on formal matters but discourages testimony on other matters on behalf of a client. This rule is outlined in the Model Rule of Professional Conduct 3.7 (1983).

For example, if an attorney is representing a client in a personal injury case and was present during the accident, they may be called as a witness to testify about what they saw. In this case, the attorney cannot continue to act as an advocate for their client in the trial.

Another example is if an attorney is representing a client in a divorce case and is also a financial expert. If the attorney's testimony is needed to explain complex financial matters, they may be allowed to testify as a witness, but they cannot continue to act as an advocate for their client in the trial.