This summary is not the rule. The rule text controls.
A lawyer usually can't be both the trial lawyer and a witness in the same case. The idea is simple: if a lawyer has to testify about facts in the case, it gets confusing for the jury and raises questions about whether the lawyer is arguing the case fairly or just backing up their own testimony. So generally, a lawyer who knows they'll need to testify has to step aside and let another lawyer handle the trial.
There are three situations where a lawyer can still act as the trial lawyer even though they might testify. First, if the testimony is about something nobody disputes, it's not a problem. Second, if the lawyer is only testifying about their own legal fees or the value of their work on the case, that's allowed too. Third, if removing the lawyer from the case would create a real hardship for the client, like if getting a new lawyer would be too costly or difficult, the original lawyer can stay on.
This rule only stops the specific lawyer who will testify from acting as the trial advocate. It doesn't automatically disqualify that lawyer's whole law firm. Other lawyers at the same firm can still represent the client at trial, as long as doing so doesn't create a conflict of interest under separate rules about client loyalty and confidentiality. If a lawyer breaks this rule, the most serious punishment they can face is a public reprimand.