This summary is not the rule. The rule text controls.
When an expert witness testifies in court, they don't have to explain all the facts or information behind their opinion before they give it. They can just state their opinion and their reasons for it. The judge can change this if needed, but normally the expert gets to lead with their conclusion first.
However, the other side's lawyer can still make the expert explain the facts and data behind that opinion during cross-examination. This means if you're questioning an expert witness, you have the right to dig into how they reached their conclusion, even if they didn't share that information upfront. This rule basically lets experts testify efficiently while still giving the opposing side a chance to test and challenge the reasoning behind their opinions.
