The seat belt retractors have a pyro device or a spring device (not sure which) which trips and pulls the belts tight in an airbag deployment situation, but it is a "once only" device, so once it fires, after that the belts can not be retracted any more and the belts are slack.

Rgds,
Chris W.
Had the passenger bag only go off in a mild front end collision on my '95 W124, belts were slack afterwards and retractors need to be replaced.

thats very interesting,you must have hit something to trigger the bag,at least you knew it worked,i was in a collision in my 86 300e,major front end collision,and the air bag did not deploy,it may have been so old that the sensor stopped working,or the powder packed inside for inflation,was so old the trigger electrodes must have been defective,ussually if a car is in a collision,the sensors need to be replace after the air bag has ben triggered,so make sure the sensor is looked at and replaced if nessecary.or it wont deploy when really needed.the sensor is only trigger after a certain mile per hour,and it senses slow down g's.if the g's are not high enough it will not deploy.generally after replacing a deployed bag,you also havew to replace the slow down g sensor.