Reuters did an article on it. Explainer: How powerful are Congress subpoenas, contempt citations?
Only a majority of the 435-member House needs to support a contempt finding for one to be reached. After a contempt vote, Congress has powers to enforce a subpoena.
How is a contempt finding enforced?
The Supreme Court said in 1821 that Congress has “inherent authority” to arrest and detain recalcitrant witnesses. . . .
For this reason, in modern times Congress has opted for a third and final approach to enforcing a contempt finding: getting its lawyers to bring a civil lawsuit asking a judge to rule that compliance is required.
Failure to comply with such an order can trigger a “contempt of court” finding, enforced through daily fines and even imprisonment, Griffin said.