To answer your question, I'll ask a question.
What is engine braking?
This engine is a heat pump. You compress air, add fuel, burn the fuel and this expanding gas pushes down the piston to creat torque to rotate a crankshaft. The momentum from that combustion helps start the process over again.
If all things are the same, and you dont add fuel and the throttle plate is closed, as the engine tries to breath and a vacuum is pulled in the intake and cylinder on the intake stroke slowing down the engine rpm. It the torque converter is locked up the car will slow down trying to overcome the resistance of the engine pulling a vacuum. The higher the r.p.m. the more brakeing is applied as there are more instances of slowing down the pistons.
So, if the throttle is open, there is going to be less of a vacuum. if the air cant be pushed out, you cant pull air in. If fuel is being added unintensionally. if the torque converter is slipping your not going to have a conection between the drive wheels and engine. If the forward gear is slipping same scinerio.
So there is a few things.