[Below you are asked to click on a link to a journal. For this to work, you need to be logged into the UW system. BUT IF OFF CAMPUS, don't click the link, but instead copy the URL given. Then open LINK TO PROXY SERVER. Next, paste the URL in the "URL to proxy" box and click login.
The most recent New England Journal of Medicine has a case study in which the patient has AV node re-entry. Normally this produces tachycardia. But in this case, the re-entry is atypical, so that the heart rate does not increase, but instead the re-entry causes the atria to contract at about the same time as the ventricles. As a result the atria are contracting when the AV valve is closed. During the atrial contraction, the pressure in the veins increases subtantially and you can see the external jugular pulse when the atria contract. This is called the "frog sign".
Observe the video. Notice the instant effect of pressing on the carotid sinus. The arrhythmia immediately stops as increasing vagal effect acts on the heart. Be sure you are clear about why pressing on the carotid sinus increases parasympathetic effects on the heart.
Here is the link to the article. (URL = http://www.nejm.org/doi/full/10.1056/NEJMicm1501617)