valve body question

I have seen on some valve bodies for sale ( usually full manual forward and reverse) I have seen them advertised engine braking in first gear. Can someone explain what that means? and is this a good thing?
Thanks
Dave

when you let of the gas the tranny holds the gear instead of freewheeling.

What do you mean by " freewheeling" sorry for the bumb question.
Dave

let off the gas in low gear, the engine goes to idle, the car will coast.

engine braking is like an old std shift deal, let off the gas the engine will hold the car back.