What is the principle of isostasy?

It is the vertical movement of the Earth’s crust as it sinks into or rises from the underlying molten mantle
The numerous plates constituting the Earth’s crust float on the fluid Earth’s mantle and dragged by underlying convection currents drift horizontally in a slow terrestrial migration known as Plate Tectonics.
Some of them are also subject to a vertical motion. Presently in places like Greenland Scandinavia or Antarctica the slow rise above the sea level is of better than a few centimeters per year. Isostasy is the motor responsible for this growth.
At any given depth and to all intents and purposes the mantle has a quasi-constant density guaranteed within the gigantic planetary crucible by the continuous blending action of convection currents.
Plates however have neither the same weight nor the same density. Oceanic plates tend to be thin and basaltic while continental plates are generally thick and granitic. Nevertheless these differences alone could not produce such vertical shift well as measured within the historical time scale. The motor acting this movement had to be somewhere else.
Gravity studies on a global level gave the key to the understanding of this mysterious event: glaciations or lack of them were responsible for the measurable Earthrise form the seas corresponding to a measurable positive increase of the gravity pull.
During glaciation times (there have been at least five during the Quaternary) huge amounts of ice accumulated on the Polar Regions. By huge I mean ice sheets several kilometers thick.
Just as the load of a ship cargo pushes down the hull and the floatation line higher and higher on the flanks of the boat this colossal amount of frozen water (thousands of cubic kilometers) sunk the rigid polar plates onto the fluid mantle underneath.
(see the upper image from Wikipedia)
As a brisk change of climate brings warmer temperatures to the Earth the polar caps melt and the water of the de-freezing ice-mountains runs into the oceans. The affected tectonic plates slowly lighten up and buoyancy laws relative to the fluid mantle make them surge from the level they had reached under the ice weight to a new ever-unstable equilibrium (lower image).
Since the climatic change and the melting of the ice is a by far faster occurrence than the vertical adjustment of the plates Isostasy continues for thousands of years after the reduction of the ice cap provoking a phenomenon whose cause is not directly visible anymore.
What is visible and a valid witness of the events is the large free-air positive gravity anomaly under the polar plates due to the compensation of the extremely dense mass of molten lava rising from the Mantle to higher levels within the crust because of their lightened burden.
If faith can move mountains as some of us were taught in catechism or in a madrasah convection currents and the laws of physics can do it too. And the two models may happily coexist without contradicting each other.