the bolt being rocked about a notional axis transverse to the barrel axis

Full Title

Breech mechanisms, e.g. locks > Bolt action, i.e. the main breech opening movement being parallel to the barrel axis > Rigid bolt locks, i.e. having locking elements rigidly mounted on the bolt or bolt handle and on the barrel or breech-housing respectively > the bolt being rocked about a notional axis transverse to the barrel axis