Why A small loop has a figure-eight pattern with two deep nulls 180 degrees apart, so you cannot tell which of two opposite bearings the signal came from. Adding a sense antenna, typically a short omnidirectional vertical, mixes its signal with the loop output in the right phase and amplitude to produce a cardioid pattern with just one null. That single null resolves the 180-degree ambiguity and tells you which direction the transmitter really is.
Watch out The idea that it increases sensitivity is backwards: combining the sense antenna with the loop actually shallows one null and generally costs a bit of signal. Its job is pattern shaping for bearing resolution, not gain or multipath cancellation.
Sense antenna makes 'sense' of which way: figure-eight plus vertical equals cardioid, one null only.