In patients with right heart failure, the right ventricle contracts poorly. Pushing on the liver (or even mid-abdomen) for about 15 seconds increases the amount of blood returned to the right ventricle.
In a normal person, the jugular venous pressure (JVP) would rise temporarily, and then normalise quickly as the healthy right ventricle pumps out the additional blood.
In a person with right heart failure, however, the ventricle is unable to do this. As a result, the JVP is elevated for the duration of the compression. This is the hepatojugular reflex.
Note: in order to elicit this sign, the patient needs to be relaxed and not holding their breath.