The Valsalva maneuver is a technique for transiently increasing the tone of the vagus nerve. (Increasing vagal tone can help to stop an episode of supraventricular tachycardia - SVT - or to stop a bout of hiccups.)
The Valsalva maneuver is performed by attempting to exhale against a closed airway. This can be done by keeping the mouth closed and pinching the nose while trying to breath out. This maneuver greatly increases pressures inside the chest cavity - which stimulates the vagus nerve and increases vagal tone.
Holding the Valsalva maneuver for five seconds or so will often terminate an episode of SVT or make hiccups go away.