Drop-in headphones refer to the headphone's design. They lack a solid bridge between the earpieces, which distinguishes them from other types of headphones. These earpieces connect by a wire and feature an extremely flexible design. The separate earpieces fit into compartments on the inside of your ski helmet. Most of the time, you'll fit one earpiece into one ear pocket, thread the wire over the rear of your ski helmet, and then slide the other earpiece into the opposite side of your ski helmet. The line that joins the earpieces is generally rather lengthy, allowing you to use the headphones with virtually any ski helmet. The presence of built-in Lithium batteries, on the other hand, makes the earbuds bulkier.