Здесь блокировка может означать "фиксатор", см. у себя по контексту.

Реконструировано на основе определения RISER CARD - Circuit board, sometimes also referred to as a riser board, that connects directly into the computer motherboard and allows additional cards to be connected into it. The riser cards were found on LPX motherboards used in desktop and some tower models. Today, newer motherboards, such as the ATX and NLX standards, are replacing older motherboards and no longer utilizing the riser card technology and instead placing the slots directly onto the motherboard allowing easy accessibility within the case.