The special affix gives a chance on hit (not on attack) to create one free, cooldown-independent Sentry, Spike Trap or Caltrops, Sentries and Caltrops summoned at the Demon Hunter's location, and Spike Traps at the location of the victim. These share the total limit with the normal counterparts, if you have any, and benefit from runes and enhancements. The chance depends on Proc Coefficient, and only triggers off damaging attacks.
Stats (Level 12)
Legendary Hand Crossbow
"This crossbow is a miracle of mechanical innovation, though its builder remains unknown. In addition to firing ordinary quarrels, the Helltrapper may also fire specialized ammunition delivering caltrops, explosive spiked mines, and even self-expanding turrets." —The Craft of War by Diadra the Scholar