How can a cross-examination of an expert on the witness stand be held if the lawyer questioning him/her has only knowledge about the law?

The lawyer would be quite useless as a lawyer. And it doesn’t matter if the lawyer understands the expert, so long as the jury does.

Consider:

1) An expert witness is useless unless what they say makes some sense to non-experts.

2) The cross-examining lawyer will have done large amounts of research and likely talking to other experts, planning what questions to ask.

3) Expert witnesses are supposed to be impartial. They’re there to offer the benefit of their expert understanding; not to actually argue about the case itself.

4) The cross-examining lawyer may not need to challenge the expertise of the witness. The lawyer is interested in the points of the case, and may be able to either get the witness to agree to make somewhat different comments than the lawyer who called the expert had them make. Or if not, they may be able to get them to bring up points the calling lawyer did not ask about, or to equivocate, or to moderate their conclusions or certainty, so that it has less impact on the case.

5) Some experts may not be great communicators. What matters is what the judge and/or jury think the expert has said, and what part is relevant to the case. So, a lawyer may be able to get the expert to say things that will confuse the other listeners, introduce doubt, or otherwise induce human reactions that help they cross-examining lawyer’s case.

Your question is not specific in its details. To begin with, the lawyer would never call an expert witness to testify without prior research regarding the expert and his field of expertise. The one exception would be if the lawyer had questions for the expert about things outside the expert’s expertise. In addition, more likely than not, even with benefit of prior research, the lawyer would question the expert on the stand such that the questions would elicit explanations from the expert in order to educate and familiarize the court (judge and jury) with those aspects of the expert’s knowledge pertinent to the case.

It’s also worth mentioning that a lawyer is never supposed to ask a question that she’s not certain what the answer will be.

A lawyer friend of mine says his job is researching the law, the situation, and the background and then explaining it to judges and juries. An expert witness shows that he is using credible informed sources.