From Lexicanum

Cyclone Missile Launcher is a variant specially designed to be fit on a Terminator Armour. Loaded with Krak Missiles and Frag Missiles, the Cyclone consists of two "cube shaped" missile boxes, twin-linked, with a targeter and sensor array. The missile and sensor unit is fixed on the back of a standard terminator armour; this fitting allows the Space Marine to use a Storm Bolter and Power Fist with the cyclone, firing both firearms simultaneously. Older designs of the Cyclone missile launcher had the sensor array hand-carried by the Terminator, thus taking away the option of a power fist but making it more accurate.¹

Sources

• ¹:Warhammer 40,000: Wargear