The phrase 'equipment clause' refers to language used in anti-slavery treaties signed between Britain and various other maritime powers in the 19th century. These treaties usually specified that the signatories considered slave transportation a crime, and that navies of any signatory nation could (and usually were bound to) stop and search vessels suspected of being involved in slaving. One difficulty with this, however, was that slavers were quite adept and determining loopholes in the various nations' laws regarding slavery. For example, many nations' laws were written such that unless a vessel's crews were caught with slaves aboard, they could not be prosecuted for slavery. As a result, slave ships would (on their runs when empty) switch their ships' flag (nation of register) to one of these nations - and if they were stopped and searched, even if they were turned over to the courts of the country whose flag they were flying, they would be promptly released. A grimmer possibility was that ships with slaves aboard being pursued could avoid prosecution by murdering the slaves and dumping them overboard so that when finally brought to a halt, they had no incriminating cargo.
As a result, Great Britain began writing 'equipment clauses' into their anti-slavery treaties. By such clauses, any ship which carried items from a specified list of equipment would be considered a slaver even though no slaves were aboard. If a ship was found to contain items such as metal manacles, excessive water storage, or excess lumber (used to build cells) it was to be considered a slaver, and the vessel and crew disposed of as slavers despite the lack of direct evidence. This practice served to both close the loopholes between various national laws that the slavers were exploiting, and in addition made the seas unsafe for slave ships no matter what flag they flew or whether they were actually carrying cargo. Eventually, slavers were driven from the seas by the late 1860s, in large part due to pursuit, seizure and prosecution under equipment clauses.
Today, various types of this legal construct still exist. The United States has just passed a law intended to combat the new technique of Latin American drug smugglers of using homemade 'semi-submarines.' When discovered, the crews of these submarines had been in the practice of simply scuttling the submarines and awaiting rescue. With no evidence, they were not prosecuted. However, the new law states that being discovered aboard this specific type of vessel is illegal and subject to high fines and prosecution.