Grabbing a bag and breathing into it repeatedly has long been recommended to ease rapid, uncontrolled breathing. The idea behind it is to increase carbon dioxide levels. Hyperventilation causes the body to expel too much carbon dioxide and "rebreathing" exhaled air helps restore that lost gas.

BUT this is a dangerous practice and should be retired.
The problem is that several medical conditions, like asthma and heart attacks, can be confused with hyperventilation. In such cases, reducing oxygen and increasing carbon dioxide can be deadly. In one case, people having heart attacks thought wrongly that they were hyperventilating and died after losing oxygen while breathing into bags.

A better way is to stay calm and practise breathing slowly and deliberately.

To Your Breathing .... Breathe !!