The beartrap is a device to enable ships to carry helicopters and allow them to land in bad weather.
A helicopter hovers over the landing pad on the deck and lowers a line with an attached probe on the end. When the line is secured to the landing pad, the helicopter is winched in. The rotors rotate at full power, which prevents the helicopter from colliding with the vessel when a wave causes the deck to rise suddenly.
Basically similar to the RAST device. (Recovery Assist, Secure and Traverse system)