From H&K's literature:
It is a Law Enforcement Modification DAO trigger system which was developed to meet the specifications of INS. The LEM system consists of a set of six drop-in parts that can be installed in all existing full and compact size USP pistols. The system uses rearward movement of the slide to pre-cock an internal separate cocking piece within the hammer. The hammer returns forward with the slide after loading or firing, yet the internal cocking piece stays cocked. The trigger pull is set to 7.5 to 8.5 pounds. With a simple spring change, this can be set at 4.5 - 5.5 pounds.
The advantage of the LEM system is reduction in trigger pull weight over other DAO systems that measure a trigger pull of 8 to 13 pounds. Since the action of cocking the hammer is completed by the slide, the LEM system eliminates any difficulty the user may experience with hammer spring resistance. The user feels only light slack and then short resistance before the hammer is released to fire the chambered round.