The Secret Treasure of Opatos is a one-use item. When used, it grants the racial ability "you are given physical damage reduction" which gives you a 10% reduction in incoming physical damage. It is unknown currently where in the process this reduction this is applied. (e.g. before PV% and dice roll or after.)
Changes in Elona+[]
At least known in Elona+ version 1.37, the physical damage reduction happens after dice roll and PV effects, but to some extent of disappointment, the amount of physical damage reduced is a single point of damage.
The 10% damage reduction is only given when you believe in Opatos of Earth as it is the passive effect form following the Opatos and therefore has nothing to do with this trait at all.
However this damage reduction can be valuable as it happens just before your HP is affected by the damage and comes in the last step of damage calculation, so it is actually not only a form of physical damage reduction.