 The KRD acts as a "mechanical sign-out sheet", physically controlling access to the key and providing accountability for its eventual return.One key (High Level/Special) is retained in a core (at 1/4 turn) until another "release" key is inserted in the other core and rotated 90° (1/4 turn).Then the "retained" key can be removed for use while the "release" key is held (trapped) for visual accountability purposes.