Due to the gag in your mouth, you can't warn the man in time as he quickly gather all the ropes into one bunch and with one fell swoop slices them all open.
The results are exactly what you expected. A huge jet of black latex shoots out of your former bonds and hits the man squarely in the chest. He screams and falls backwards, doing a soumersault and landing on his back, the huge, pulsing mass of black latex covering most of his chest already rapidly making its way up his neck and down his stomach. He screams and manages to get out one beg for help before the latex pours into his mouth and covers most of his face, leaving him thrashing and mumbling on the floor.
You quickly jump to your feet and back away, hand over your mouth, as the man becomes engulfed in so much latex he looks like a giant, squirming black grub. You take a few more steps towards the door as the latex slowly begins to tighten around his frame. His arms and legs are slowly pulled into his rapidly thinning body which grows long in length and thick in girth. It rolls around on the floor like someone rolling a lump of black Play-Doh on a table as it gets thicker and thicker around until it's about nine feet long and two feet around.
The front of the tube-like shape that used to be a man thins slightly and becomes sort of triangle-shaped. Two green dots of latex suddenly appear on the opposite sides of it and it suddenly opens a mouth, exposing soft, rubbery white fangs and a spongy forked tongue that drips latex. The back end things as well until it forms sort of a tail. The large black snake opens it's mouth again and hisses at you, spraying little drops of white latex venom on the ground.
You are snapped out of your trance by the snake rapidly slithering towards you on the floor, occasionally hissing and rattling it's tail like a rattlesnake. You turn and see the door about ten feet to your right. Why didn't you run while the poor sap was transforming?