Feruchemy

A second magic of Scadrial in which a person uses a worn metal object — a metalmind — to store an attribute of themselves and draw it back later. A Feruchemist who stores weight in iron grows lighter as the metalmind fills, and can draw the stored weight back when it serves them. MB2:AoL:Prologue

The Terris art practised by the Keepers — a metal-storage discipline whose attributes include strength, sharpness of sight, recent memory, weight, and even age itself. Pay one end, draw the other. Sazed now suspects that a person who is both Feruchemist and Allomancer can burn their own filled metalmind as a new Allomantic metal — releasing its stored attribute many times over. That, he believes, is how the Lord Ruler stayed young for a thousand years. MB1:TFE:Epilogue

The Terris art practised by the Keepers. Unlike Allomancy, Feruchemy does not burn metals away — each piece of metal is used as a reservoir. A Feruchemist diverts some bodily attribute — strength, sharpness of sight, recent memory — into a metalmind, paying it down at one end and drawing it back at the other. What attribute a metalmind can carry is fixed by which metal it is and how much of it. The art is the reason Terris dress is heavy with rings and earrings and bracers. MB1:TFE:Ch 22