The principle
An OTDR fires laser pulses and times the faint Rayleigh backscatter returning from every point of the fiber, plus Fresnel reflections from index steps (connectors, breaks, the end). Distance = (time × light speed in glass)/2. The result is the trace: a downward-sloping line whose slope is dB/km, punctuated by events.
The three knobs
Pulse width: short = fine resolution, short reach; long = far reach, big dead zones. Range must exceed the fiber; averaging time trades speed for a smoother trace. Auto modes are for orientation; certification shots are set manually.
Dead zones and launch fibers
After each reflection the detector is briefly blinded — the event dead zone (where two events merge) and attenuation dead zone (where loss can't yet be read). This is why a launch fiber before the link and a receive fiber after it are mandatory: they move the first and last connectors out of the blind zones so their loss can actually be measured.
Reading events
Reflective spike + loss = connector or mechanical splice. Non-reflective step = fusion splice or bend. A huge reflection then noise = the end (or a break). A ghost is a repeat reflection at a multiple of a strong event's distance — it has no loss and disappears when you change pulse width. An apparent gainer at a splice between dissimilar fibers is not free power: shoot both directions and average.