Electromechanical lock and its operation method. The lock includes a power transmission mechanism (102) to receive mechanical power produced by a user of the lock; a generator (104) to produce electric power from the mechanical power; an electronic circuit (108), powered by the electric power, coupleable with a key (112), to read data from the key (112), and to issue an open command provided that the data matches a predetermined criterion; an actuator (116), powered by the electric power, to receive the open command, and to set the lock in a mechanically openable state; and a threshold device (100) to control the power transmission mechanism (102) so that a mechanical tension rises until a predetermined force threshold is exceeded, whereupon the mechanical tension transforms to an action producing the mechanical power received by the power transmission mechanism (102).