ALT OVERSHOOT ON DSCNT.

Narrative

DURING DSCNT TO BNA; 7000 FT; THE AUTOPLT DID NOT CAPTURE SAID ALT. MY TECHNIQUE FOR DSCNT IS; USE IAS HOLD BUT AT 1000 FT AND NOT LESS THAN 500 FT PRIOR TO DESIRED ALT; TO SELECT VERT SPD; REDUCE THE RATE BY 1/2 AND THEN ENSURE THE ALT IS STILL ARMED. I FEEL THE S80 AUTOPLT IN IAS HOLD/AUTOTHROTTLES ON; GIVES EXCESSIVELY ABRUPT LEVELOFFS AND PWR ADVANCES. REDUCING THE RATE LESSENS THIS. ON THIS FLT DURING DSCNT INTO BNA; I USED THIS TECHNIQUE; TO INCLUDE VERIFYING THAT THE ALT WAS STILL ARMED. BTWN 500 FT ABOVE AND THE DESIRED ALT OF 7000 FT; THE CAPT WAS DISCUSSING MY METHOD OF KEEPING THE ICING LIGHTS OUT. CONSEQUENTLY; I MISSED OBSERVING THE CAPTURE AND LEVELOFF AT 7000 FT. WHY THE ACFT CONTINUED BELOW 7000 FT I DO NOT KNOW. THE ALT WAS ARMED AT 7500 FT. HOWEVER; WHEN I NOTICED THAT THE ACFT HAD DSNDED BELOW THE ASSIGNED 7000 FT ALT. I IMMEDIATELY DISENGAGED THE AUTOPLT AND AUTOTHROTTLES; ADDED PWR AND PULLED ON THE YOKE. SHORTLY AFTER BEGINNING THIS MANEUVER THE ALT WARNING WENT OFF. THE ACFT DID NOT DSND BELOW 6700 FT ON THE CTR ALTIMETER; AT ANY TIME DURING THIS MOMENTARY DEV. THE SUBSEQUENT LEVELOFF AND REST OF THE APCH WERE VISUALLY HAND FLOWN BY ME. ATC DID NOT MENTION THIS ALTDEV. THE CAPT AND I DID NOT DISCUSS THIS EVENT UNTIL HE HANDED ME HIS VERSION IN WRITTEN FORM.