The same type of wrist ties that you can see binding the hands of captured prisoners in TV shows, movies and even occasionally on the news. These are made of the same sort of material and look exactly like the real thing.
Unlike other sets, these do indeed have functional one-way locks which means they really do bind. For health and safety reasons please be careful with binding of wrists as this will compromise a players ability to respond to loss of balance, tricky terrain and other emergency situations.
The one-way locks are actually very secure so please be aware that it can be fiddly to remove and so do not restrain anyone unless they expect to stay that way for quite some time.