Skill practiced on dummies, for short

CPR

This life-saving technique often involves chest compressions in pairs of thirty.

This clue hints at a lifesaving technique often trained on mannequins (called "dummies" here). The phrase "for short" signals the answer is an abbreviation—CPR stands for cardiopulmonary resuscitation. It’s a common term in first aid, so the clue plays on both the training method and the shorthand name.