A mechanical watch's balance wheel oscillates back and forth; the escapement delivers an impulse on each pass through the centre rest position. In a perfectly poised balance, the time between the forward-pass impulse and the backward-pass impulse is exactly equal; the watch ticks with a single uniform sound at exactly the watch's beat rate (4 ticks/sec for 4 Hz, 8 ticks/sec on a timegrapher counting half-beats).
In a poorly poised balance, one impulse arrives slightly earlier than the other; the audible result is a "tick-tock" double-pattern instead of a uniform "tick-tick-tick". This is the same audible difference between a properly-set metronome and one that is slightly off; the human ear detects asymmetry above ~1 ms reliably. On a timegrapher, beat error displays as two parallel rate-trace lines at slightly different angles instead of one; the vertical separation is the beat-error magnitude.
"If the watch ticks like a metronome, you have nothing to worry about. If it ticks like a horse trotting, send it for service."- Watchmaker on diagnosing beat error by ear
Modern watchmaking standards: a freshly-serviced movement should show 0.0-0.4 ms beat error. 0.4-1.0 ms is acceptable for most production watches and not user-detectable. Above 1.0 ms the asymmetry becomes audible; above 2.0 ms the asymmetric impulse can cause the balance to stall in unfavourable positions (the early impulse barely sustains oscillation while the late impulse overshoots). Watches with persistent beat error above 2 ms typically have a service issue or a damaged hairspring.
Correction requires repositioning the hairspring relative to the balance: the hairspring stud (the fixed end attached to the balance bridge) can be rotated, or the impulse pin (the pin that engages the escapement) can be moved. On modern free-sprung balances the adjustment is via a dedicated eccentric stud carrier; on index-regulated balances the watchmaker must release the stud, rotate, and re-set. Either way the operation is delicate and is part of the regulation step at every service.
For buyers and owners, beat error is one of three numbers on a service hand-over printout (rate, amplitude, beat error). A clean handover shows beat error 0.0-0.4 ms across all positions; persistent beat error >0.5 ms suggests the watchmaker rushed the regulation. Audible double-tick on a watch you own is the first sign of a service-due condition; combined with low amplitude it is unambiguous evidence the watch needs attention.