This summary is not the rule. The rule text controls.
Here's a plain English summary of this rule:
The judge running your trial cannot also be a witness in that same trial. This makes sense when you think about it. A judge is supposed to be neutral and decide the case fairly. If the judge also testified about what happened, that would mix up two very different jobs: deciding the case and being a source of evidence in it.
This rule protects the fairness of the trial. It stops any confusion about whether the judge is acting as a neutral decision-maker or as someone with personal knowledge of the facts.
One important detail: you don't have to object in the moment if this happens. Normally, if something improper happens at trial, your lawyer needs to speak up right away or you might lose the right to complain about it later. This rule is different. Because a judge testifying is such a serious problem, the law automatically protects your right to raise the issue later, even if no one objected at the time.
