A metric modulation is a tempo or meter change that uses a shared rhythmic value as the bridge — the new section keeps something from the old section, even though the pulse changes. The three pivot types (quarter constant, 8th-becomes-quarter, triplet pivot) are built from scratch in the prerequisite Metric Modulation lesson in the Jazz Fusion track; this lesson is about how metal uses them.
The mechanical part is straightforward: the drummer plays the pivot value continuously across the bar boundary, then re-feels the meter once the pivot has carried them through. The hard part is making it sound like a deliberate musical choice rather than a glitch. The cleanest modulations have a rhythmic flag — a fill, an accent, a snare hit — that points at the pivot moment. The audience hears the change and accepts it; the band hears the pivot value and stays together.