What happens when a larger PCIe card is inserted into a smaller PCIe slot?

When a larger PCIe card is inserted into a smaller PCIe slot, it is referred to as "down-plugging." This process occurs because PCIe slots are designed to accommodate varying sizes of cards, such as x1, x4, x8, and x16. A larger card, for example an x16 card, can physically fit into a smaller slot like an x1 or x4 due to the nature of the PCIe design, which allows backward compatibility.

During down-plugging, while the larger card can be inserted into the smaller slot, it will operate only at the bandwidth capacity of the smaller slot. This means that even if the card has more pins and a wider bus designed for higher data transfer rates, its functionality will be limited to that of the slot into which it has been inserted.

In contrast to down-plugging, other terms such as up-plugging, cross-plugging, and stacking do not accurately describe this scenario. Up-plugging typically refers to inserting a smaller card into a larger slot, which is a common practice without any loss of capability. Cross-plugging generally refers to connections between different types of expansion interfaces, and stacking refers to a physical arrangement of components rather than