From the Document: "Congress's spending power gives it substantial control over the activities of executive branch agencies. For instance, through appropriations riders, Congress can direct agencies to engage, or to refrain from engaging, in particular activities in order to receive appropriated funds. Over the years, Congress has inserted several firearms-related riders into appropriations bills. Perhaps the most well-known riders are the Dickey and Tiahrt Amendments, in reference to the Members of Congress who first advanced them. Still in force today, these riders restrict agencies within the Department of Health and Human Services (HHS) and the Department of Justice (DOJ) from engaging in certain activities related to federal firearms regulation."
CRS In Focus, IF11371
Congressional Research Service: https://crsreports.congress.gov/