Merry Christmas to all.

Can anyone help me with this: In Arena GUI, for instance, there is an option were you can limit the strenght (ELO) of the engine you are playng against. For instance, if I play against Houdini 3 and set ELO strength to 1900, is there any realism to this in terms of validity and reliability?
i guess the confidens intervals can be very large. And exactly by what means or mechanism is the reduction of strength achieved? Furthermore, is this option implemented differently between GUI's or between different engines?