The important thing first: Metal Needle is banned in World Beyblade Organization X Format play. It is the only part on that ban list, and the reason is physical rather than competitive — the metal tip dents Beystadium floors, particularly when the bey is launched from height and drops onto the surface. Individual organisers can restore it through an opt-in banlist variation, so you will occasionally see it legal at a specific event, but the default assumption at any WBO tournament should be that it is not usable.
Mechanically it is a conical sharp tip made of metal instead of plastic. The narrow point and hard material produce very little friction against the floor, so a combo parks in the middle of the stadium and grinds out a long spin, with the added mass down at the very bottom of the build helping it stay planted.
Outside sanctioned play it slots onto defence and stamina blades. Where it is banned, the substitutions are Needle or Point, which chase the same low-friction centre-holding behaviour in plastic.