postgresql - What is the use for Auto FK index in pgAdmin? -

when creating foreign key constraint in postgresql from pgadmin (1.12.2 in case), following option checked:

auto fk index 

i know if it's right leave checked time, , understand how overhead works.

for instance, following constraint:

alter table "user"   add constraint fk_user_region foreign key (intregionid)       references region (intid) match simple       on update no action on delete no action; 

creates following index:

create index fki_user_region   on "user"   using btree   (intregionid); 

note creates index only when creating constraint from pgadmin.

there not documentation pgadmin, , nothing option.

thank you.