Source : WordNet®

fall back
     v 1: fall backwards and down
     2: hang (back) or fall (behind) in movement, progress,
        development, etc. [syn: {lag}, {dawdle}, {fall behind}]
     3: move back and away from; "The enemy fell back" [syn: {recede},
         {retire}] [ant: {advance}]
     4: retreat [syn: {lose}, {drop off}, {fall behind}, {recede}]
        [ant: {gain}]
     5: have recourse to; "The government resorted to rationing
        meat" [syn: {resort}, {recur}]
     6: go back to bad behavior; "Those who recidivate are often
        minor criminals" [syn: {relapse}, {lapse}, {recidivate}, {regress},
         {retrogress}]

Source : Free On-Line Dictionary of Computing

fall back
     
        A feature of a {modem} {protocol} where two modems which
        experience data corruption, e.g. due to line noise, can
        renegotiate to use a lower speed connection.  See also {fall
        forward}.