Trapping was an expression for clock watching. You remember all those folks who would be out of overalls already at the end of the shift, all togged up ready to rush out of the door as soon as the hooter sounded? Like greyhounds in the trap. Trapping was originally just getting ready to go home while still on the firm's time.
Back in BA Overseas Division there was an (unofficial) "Trap Roster" When you were at the head of the roster you went home early if the work load was light. Seems it was a tradition that went back to the days of Orville and Wilbur.
I once saw a picture of an interesting method Avro used to prevent trapping during the forties. Just in the interests of saving space of course, they had coat hangar rails instead of lockers. When you had hung up your coat and put on your overalls the coat hangar rails were winched up to the roof out of the way. At the end of the shift they were winched down again so that you could retrieve your coat and hang up your overalls. No trapping at Avro it seems!
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Through difficulties to the cinema