By Rasher
A carefully concealed set of iron manacles hidden just above an opening through which something shiny can be seen within reach. A gap in a treasure box, the space between the bars of a vault door, the cracked window to a noble’s house etc.
Trigger: When something slips in to the opening and reaches about halfway to the bait, or touches anything on the other side.
Detect: Per based Traps -2 to see the hidden mechanism inside.
Disarm: DX based traps to cause the manacles to drop down, allowing the player to take the bait
Circumvent: Lockpicking (at -2 if the cuffed subject has to lock-pick it themselves in order to escape)
Evade: Hearing allows you to make a dodge roll (to pull your hands back when you hear a click) An Escape roll at -4 (to slip them off you, bonuses for double jointed, flexible etc.) or an absurd contest of bare handed thrusting damage vs the handcuff's stats (See notes)
Effects: a metal bar with a set of handcuffs built into either side of it slides down, the manacles landing on the wrists of the target and locking shut with a click. The target can neither reach the bait nor pull their hands back. Targets who only reached in with one hand are still caught, but do have a hand free.
Shots: 1
Rearm: Yes, DX based traps
Steal: You can steal the bait inside the trap, or a pair of iron handcuffs
Notes:
The handcuff's chain has a DR of 6, an HP of 4 and an effective tensile ST of 30 (GURPS High Tech Page 56)
Variants
You could obviously make the handcuffs out of stronger materials than iron (see high tech or low tech for details), you could also couple this trap with another one to incapacitate the captured subject with some sort of knock out gas or other hazard if you wanted it to be even nastier but really the point of this trap is to hold the would-be thief in place long enough for a guard to come by and notice them (or forever if the area is abandoned). You could also create a higher TL version of this trap by using inferred sensors and a higher tech restraint, if so Hacking may be used instead of Lockpicking to circumvent.
Stockade Trap
Effectively the same trap, only instead of a set of handcuffs coming down a over the hands a stockade locks over the head and hands, in this variant the trap has an opening wide enough to crawl through or climb through that warrants the victim pulling up and in to it, trapping them in place far more effectively than handcuffs would (Escape -6 to Slip out of)
Foot Manacle Trap
This version is on the floor and cuffs a victim who steps on to the small trap door to an iron chain around the ankle, but is effectively the same trap, with the victim being chained to the floor itself. This is useful in the sense that it has no obvious bait and is much less space consuming than a trap door or pit trap. (Escape -5 to slip out of)
Adventure Ideas
Pretty much useful for any dungeon, heist location, ancient temple or anywhere else that would want to capture thieves, the real advantage of using a trap like this is that it works in basically any setting, from mundane to fantasy to horror. This is also especially fun to use on characters with the Kleptomania or Greed disadvantages as you can get them to make a self control roll to avoid trying to reach in and grab that shiny treasure, only to put them in a very vulnerable position with an enemy not too far away.