Bob, it is in fact a "torsion spring" and the best place to see it is "You Tube". There are many video's there Under "Sigma Trigger". Removing it seems to slightly lighten trigger pull. I have not heard of anyone having any issues after doing this.
I agree with leejack. Leave it in there. It is a double acton trigger. It is supposed to be stiff. Learn to use it as intended. A little polishing at the mating surfaces was all mine required. From what I have read about the pigtail spring it is in there to insure that the trigger resets after firing +P loads. Often the guns works fine with it out, but not always.
I agree with Leejack and Mitchellb. You will have to remove the pigtail spring to polish the sear cam and it will be a pain in the ... to replace but do replace it. The slight improvement in trigger weight is not worth the potential loss of reliability. Furthermore, in the off chance of of an accident or justifiable shooting you do not want to have modified the firearm. If the trigger is smooth the weight will not be an issue.
After reading the patient of the Sigma I felt that it was OK to remove the pigtail(torsion) spring. The patient states that is there to backup the two coil reset springs in case they were to fail.
It also will wear a groove in the plastic housing that it is in, so that it will have to be replace in the future.
Mine is not used for SD.