How can rescuers ensure that they are providing effective breaths when usimg a bag mask device? ?

A bag-mask is a medical device designed to ventilate a patient in respiratory arrest.

It must be on hand in an emergency department of the hospital or an ambulance.

It can be used before or after intubation while waiting for a mechanical respirator.

It can be used in CPR.

The bag-mask is preferably connected to a source of oxygen.

To use it requires 2 hands:

The right-hand holds the ball and exerts pressure with the 4 fingers to push the air (press with the thumb is an error).

The left hand applies the mask to the mouth and the nose, keeping a good seal, then putting the hand on the chest of the victim to check if there is an expansion/shrinkage of the rib cage due to the entry of the air.