In pericardial tamponade, blood escapes from the heart but is trapped within the pericardium, so pressure builds up between the pericardium and the heart, compressing the latter and interfering with its pumping.
In cardiac compression, blood escapes from the heart but is confined within the pericardium, resulting in increased pressure between the pericardium and the heart, which compresses the latter and hinders its pumping function.