§ 973 [Waiver of Spousal Testimonial Privilege]

Here's a plain English summary of this rule:

If a married person chooses to testify in a case where their spouse is involved, or testifies against their spouse, they give up their right to refuse to testify in that same case. In other words, once someone voluntarily takes the stand under these circumstances, they can't later claim the privilege to stay silent about the same matter. This doesn't apply if the person was wrongly forced to testify in the first place.

This rule also removes the privilege in civil cases where a married person is suing or defending a case that benefits their spouse, or benefits both spouses together. So if a husband or wife brings a lawsuit that helps their spouse financially or legally, they can't use the marital privilege to avoid testifying about it.

The main idea is that this privilege can be given up, either by choosing to testify or by being involved in a case that helps your spouse. The law does not let someone have it both ways by testifying when it helps them but staying silent when it doesn't.