These lockable handles are used for locking a garage door when an operator is not part of the picture.  When the T-handle is twisted on the outside face of the door, it pulls a metal finger away from a locking channel located on the inside of the vertical rail(s).  It will typically reengage when the door is lowered.  There are a couple of problems with these handles.  One, if an operator is installed and the locking mechanism is not disabled, the operator will pull the locked door until damage is done.  Two, these handles are not designed for lifting the door.  Lifting the door by these handles will often crease the door skin because the handle is not attached to an adequate support.