Lock-in amplifiers are very useful when one can stimulate a system at a fixed frequency. They are capable of amplifying weak response in the presence of noise. In the simplest case, they consist of an AC amplifier followed by a synchronous detector which operates at the frequency of the stimulation. commercial lock-ins cover a wide fequency range and can have narrow band filters tuned to the frequency of stimulation. Often, even a bare-bones lock-in can provide impressive results when compared with simple amplification (microvolt or better sensitivity in the presence of broadband noise or DC fluctuations).
In the present case, the stimulus is an AC current (or DC modulated at a specific frequency) applied to the suspect power rail. A small loop pickup would have an induced voltage if it is near a circuit board trace carrying that current. The induced voltage (which may be only microvolts) is AC amplified by a large factor and input to a syschronous detector, driven at the stimulus frequency. The detector output would have a DC level proportional to the induced voltage (note that there is a phase dependence and quadrature detection can be very useful in some cases).
Note that this is not limited to a loop pickup. A hall effect sensor would work and even simply probing the trace voltage level might suffice.
Commercial lock-ins are general purpose instruments capable of operating over a wide frequency range and with filters designed to enhance sensitivity at the stimulus frequency. If the stimulus frequency is fixed, the instrument is much simpler.
