Close. When you let off the throttle on a diesel, the air in the cylinders acts as a spring and pushes the piston back down, providing little to no braking. A engine retarder, (compression brake, engine brake, call it what you will) opens the exhaust near the top of the compression stroke so there is no spring effect on the piston, theres just resistance on its way up providing braking. The popping you hear is the compression being let out.
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1995 Cummins 12v Reg. Cab 4x4 5 speed, 250,000 miles, boost/pyro, straight piped 5" stack, #10 plate, pump cranked, coal rollin.
