Marc Daniel Nelson breaks down pre-delay as the time gap between a dry vocal and the moment its reverb kicks in, and explains how that gap is what creates a sense of three-dimensional space in a mix. Push the reverb back in time relative to the vocal and the vocal snaps forward, sounding present and upfront without losing the sense of size or atmosphere around it.
Nelson skips the BPM-division math and works by ear instead. His two go-to starting points are 75ms and 120ms, which he treats as rough anchors to adjust from depending on how much separation the song needs.
The demonstration uses a vocal from Laura Lewis's "Old Gray Pine" to let the effect speak for itself. With pre-delay dialed in, the reverb sits behind the vocal rather than merging with it, giving each element its own plane in the stereo image. Nelson's point is that even a basic stock reverb plugin can sound expensive and record-like once the pre-delay is doing that separation work.