Totem, Voltus (or VoltusFi), or any other EM / IR drop analysis tool simulates current flow in R or RC network (for power nets or signal nets).
When it does dynamic EM / IR simulation, a current through each parasitic resistor is a function of time.
For each resistor, the tool calculates average current (over time), RMS current, or peak current.
These values are then compared with the critical current density values, different for each of these modes, and which may be complex functions of various parameters (such as temperature, metal line length, etc.).
A percentage of violation (or similar characteristic) is reported by the tool.
This is a top level description of how EM analysis is performed.