"Lowering the focus" means putting the focal point slightly below the material surface instead of on it. It is a real technique with specific uses, not a general improvement.
When it helps
- Cutting thicker material. Focusing a third to halfway into the thickness gives a more parallel kerf through the depth, instead of a wedge that's wide at the top.
- Reducing top-surface charring, because peak intensity is no longer at the surface.
When it hurts
- Engraving. You want the smallest spot exactly at the surface. Defocusing softens detail — which is why a slight defocus is deliberately used on glass to avoid chipping, and is a mistake almost everywhere else.
- Fine detail cutting, where a wider surface kerf loses small features.
How to find your own number
Cut the same test part at surface focus, then 0.5mm, 1.0mm and 1.5mm lower. Snap each one and look at the edge: you want the squarest section with the least charring. Note the number against the material and thickness in your settings sheet — it does not transfer between materials.
LaserPecker publish their own documentation on this topic — worth reading alongside these community notes.
Official article