B737-300 FLC HAS ALTDEV.

Narrative

WHILE ENRTE TO PHX; ATC GAVE US A CLRNC TO CROSS MOHAK AT FL250. MY CAPT ENTERED FL250 IN THE MCP; WHICH I VERIFIED; AND I ALSO CHKED THE FMC DISPLAY TO ENSURE FL250 WAS ENTERED FOR MOHAK. UPON DOING SO; I VACATED FL270 AT THE PROPER TIME IN ORDER TO MAKE THE REQUESTED RESTR. DURING THE DSCNT; I WAS MONITORING THE ACFT'S APCH TO FL250 AND I DECIDED TO REDUCE THE RATE OF DSCNT BY MANUALLY APPLYING REARWARD PRESSURE TO THE CTL COLUMN. AFTER ATTAINING THE DESIRED RATE OF DESCENT; I ALLOWED THE ACFT TO CONTINUE ITS DESCENT WITHOUT MANUAL INPUT. I THEN DIVERTED MY ATTN TO THE FMS DISPLAY; AND WHILE DOING SO; I HEARD THE ALT ALERTER TONE. IMMEDIATELY NOTICED THE ACFT DSNDING THROUGH FL250 AND I REACTED BY PULLING ON THE CTL COLUMN AND DISENGAGING THE AUTOPLT. I WAS ABLE TO LEVEL OFF AT FL245. IMMEDIATELY UPON DOING SO; ATC CLRED ME TO FL240 WHICH I DID WITHOUT FURTHER INCIDENT. (ATC NEVER QUERIED THE INCIDENT NOR COMMENTED ON IT.) I WAS NOT AWARE OF THE FACT THAT A MANUAL CTL COLUMN INPUT CAN CAUSE THE AUTOPLT TO NOT CAPTURE AN ALT. I ALSO SHOULD NOT HAVE DIVERTED MY ATTN DURING THE LEVELOFF.