The operating principle of HDD DTH drilling tools is similar to that of conventional down-the-hole systems. A pneumatic or hydraulic hammer is connected to the front end of the drill string, while the rear end is driven by a rotation unit. During drilling, the hammer advances to the bottom of the borehole, where compressed air or hydraulic oil flows through the drill rods into the hammer. The internal air or oil distributor activates the piston, delivering high-frequency impacts to the bit and fracturing the rock. At the same time, the rotation unit turns the drill bit, combining percussive action with cutting forces to achieve efficient rock breaking. The resulting cuttings are then flushed out of the hole by compressed air or circulating drilling fluid.