The other day I was stern-too, quite solidly and when I dove on my anchor it was just sitting on the sea floor, sideways not dug in.
I had backed down on it at 1500RPM on two 57HP engines!! But if it wasn’t set (dug in) what was holding us in place?
Especially Stern-too, because the anchor is always downhill from where the boat is, but even in a flat bay there is a tremendous amount of holding power in your chain and Anchor and its friction against the bottom.
50M of 12mm chain weighs approximately 170Kg, Add, in our case 40Kg of anchor, a couple of Kg’s of shackle etc and you have over 210Kg of weight bearing down on the seafloor. A good way to imagine this is think of 50M of chain laid out on grass and you trying to pull it through the grass as a gym exercise…. good luck with that. You’d need to apply more than 300Kg of force to get it moving because of the coefficient of friction.
Of course its even harder to pull up hill toward a stern-too mooring.