The Jacobs Engine Brake functions by holding the exhaust valves open when the piston would normally be compressing the air/fuel intake charge, preventing the engine from producing positive power.
There are settings that need to be maintained to ensure proper engine brake performance. It is similar to performing a valve adjustment.
Do a web search for Jacobs Engine Brake Adjustment, and you should find pdf versions of the service manuals for this.
It has been a ling time since I did any of this kind of work, but I think there are also some gages and setting tools available to make the job easier to perform. And when they are adjusted properly they will perform at their best, but as BW9 pointed out, they didn't work as well back then.
The engine brake systems on today's modern electronic engines work much better than the original Jacobs Engine Brake. Also, the name "Jake Brake" has become synonymous with all engine brakes, and many people call the engine brake on their truck a "jake" even though it may not a Jacobs Engine Brake. I have not seen an actual Jacobs Engine Brake on any of the newer electronic engines, save for an early Detroit DDEC Series 60 engine. I think that the Cummins N14 engines could be fitted with them too.
I have even heard guys talking about putting "jakes" on their Dodge pickups with the ISB Cummins engine, but all those are is a mechanical valve that blocks the exhaust pipe downstream of the turbocharger.