There is a one-way mechanical roller clutch in the transmission that allows the drivetrain to overrun the engine rpm when decelerating.
When the shifter is placed in drive, second & low, the Overrun Clutches are applied. This effectively locks the mechanical roller so that it cannot overrun, causing the engine compression to decelerate the vehicle.
That being the case, your Overrun clutches may have expired. There are only two and they are relatively small in diameter as well. If you use them a lot, they may have worn.
Some additional background information would be really helpful; i.e. how many miles are on your trans, is it still stock or modified, etc.