Seekers multiply the inherent chance-to-be-available-for-pick of the relevant enchant types by 6. This only applies to enchants received from enchant containers, not those just laying on the ground in stash rooms.
Right now there are 21 base enchant types that can be received from an enchant container, 4 of which are torso enchants. Of those, resilient-form has an availability chance of 100%, and acid-gills, cloak-of-shadows, and risktaker all have 30%. The majority of other enchant types have < 100% chance (but every slot has at least one with 100%); as it turns out currently the "total availability" is 976. Each base enchant type that has been unlocked and passes its availability roll is put in a list, and the base type of the enchant actually granted is selected at random from that list; seeker has no direct impact on that choice.
So, assuming for simplicity that all base enchant types are unlocked and all the non-torso types have 100% chance (they don't, but for simplicity) :
- with no seeker the average chance of getting a torso enchant would be 190 / 976 ~= 19.5%
- with torso-seeker the average chance of getting a torso enchant would be 300 / 1186 ~= 25.3% (i.e. ~30% increase in the chance)