Pretrigger refers to the portion of a captured waveform that happened before the trigger condition was met. Digital oscilloscopes achieve this by continuously recording data into a circular buffer and stopping the recording a set time after the trigger. This allows engineers to see the exact cause of a glitch or fault, rather than just the aftermath.
Heard on the job
Set the scope to 50 percent pretrigger so we can see what the power rail was doing right before the voltage dropped.