The Reaver Jetbike chains are just for looks. They don't really do anything.
If you want, you can put them on a Raider to represent slave snares, but I think they're a little small for that.
Slave Snares are described pretty well in the Codex, but since you're confused it works like this.
You pay for the Vehicle upgrade once, and then represent with an arbitrary number of snares.
Then, when you fly over an enemy unit, that unit suffers D6 S4 hits. You don't have to roll to hit, they're already hits. You just have to roll to see how many hits there are, then roll to wound, and then your opponent makes his armor saves.
Then anyone who dies is automatically taken captive. Bwahaha.
AX