On some types of ESU (guess which type - there are 'Floating' and 'Earth-Referenced' @ HF) you sometimes need to "earth-reference" the 'neutral' side of the meter to 'tie' the ESU dispersive to the ESU tester earth, at HF, (the ESU tester earth should be,connected to the ESU earth directly) to compensate for the imbalances in the leakage paths that 'present themselves', at HF, to the active and dispersive circuits within the test instrument and the effects this has on dispersive voltages due to leakage wrt Earth - those significant leakages that occur at higher power settings.
The capacitor effectively 'ties' the dispersive, at HF, to the earth of the tester and ESU. Any active leakage currents are also referenced to this earth (and that of the ESU which should be connected to the earth terminal supplied on the tester). For testing purposes you need to earth-reference the ESU dispersive at higher power settings because of the relatively significant imbalances in leakage paths, within the ESU leads, and the ESU tester, that cause ESU alarms when active current is not equal to dispersive current and when the leakages cause the dispersive to 'float' wrt Earth. The relatively low leakages at lower power settings do not cause problems.
The earth-referencing capacitor prevents these alarms from occurring at higher power settings and under conditions where there can be significant imbalance in active and dispersive circuits WRT earth and significant dispersive voltages as a result. This prevents problems on ESU's that monitor imbalances between active outputs and dispersive return currents within the test circuitry, that is earth referenced internally, and the dispersive voltage that would otherwise 'float' WRT Earth. otherwise imbalances in leakage between the active circuit and dispersive circuit cause difficulties at high power settings in particular.
Surely the use of this method is common knowledge to anyone purporting to be a 'biomed' that has used any form of ESU tester.