to automatically retract the blade after cutting , a flat serpentine spring is integrally formed with a thumb piece which is slidably received in a slot formed along the upper edge of a housing / handle . the housing has a finger rest projection on the lower edge to facilitate cutting while the blade is projected with the thumb . the two halves of the housing snap together using a block - like projection formed on one half , and a pair of tabs formed on the other . the tabs , which are resilient , slide along the sides of the projection until recesses on each of the side of the projection are reached . inwardly extending ridges on the ends of the tabs enter the recesses to provide a click lock . an aperture between the tabs allows pressure to be applied to the top of the projection to separate the two halves for blade reset / re - adjustment . the blade carrier , which is adapted to support two different types of blades , is slidably mounted in the housing so that it and the thumb piece are free of detents or click - locks . the carrier &# 39 ; s upper and lower edges have a number of apertures to permit setting in different orientations and adjustable positioning with respect to an engaging projection on the base of the thumb piece . this , and the ability to set at least one of the blades in a plurality of different positions on the carrier , allows blade projection adjustment .