How is the temporal profile of a wave signal obtained via its overlapping with a pulse signal?

In a thesis I am reading, it is stated that:

"The second beam is used to detect the time profile
of the THz waveform. This is achieved by temporally overlapping the much longer
THz pulse with the very short detection pulse. The difference in pulse durations allows
one to discretely sample the terahertz temporal profile by varying the path lengths
with an optical delay line."

The second beam is a reference to the three beams: generation, detection and excitation, needed for the Time-Domain Spectroscopy.

But how can a difference in the duration of pulse allow for the temporal profile of the signal?

I have seen similar temporal profiling being used in articles such as this, but I still can’t visualise how extending the path of one of the signal allows for the profiling of the signal being analysed.