What it means
Jaimini astrology keeps a separate point for marriage called the Upapada, also known as the Upapada Lagna. It is the arudha pada of the 12th house. Think of it as the image of the partnership, not the partner.
The planet that rules the Upapada's sign is read for how that image holds up. This rule looks for two warning signs. The first: the Upapada lord stands in the 6th, 8th or 12th house counted from the Upapada itself. The second: it is debilitated in the birth chart. The rule passes when neither is present.
About 2 charts in 3 pass, so the chart that fails is the more interesting one. Treat a pass as a lack of two classical warnings, not as a promise. Treat a fail as a point to read further, not as a verdict. Jaimini texts look at several more factors.
The Upapada is computed for you. It uses the BPHS method, including the exception for the 1st and 7th positions. That calculation is the hard part. The rule on top of it is short.