A Yagi antenna uses a gamma match. The adjustable gamma rod connects to:

The gamma match works by tapping the driven element at a point where the impedance matches the feedline. An adjustable rod runs parallel to the element to create this connection. Because this parallel rod acts like a short transmission line, it introduces unwanted inductive reactance into the system. To cancel this inductance, a variable capacitor is placed in series between the feedline and the rod. The RF signal travels from the coaxial cable's center conductor, through this variable capacitor, and then into the adjustable gamma rod.