The kit sear spring is stronger than the factory spring and makes the sear reset more reliably. You really should use it. That yellow pin is used to temporarily hold the replacement trigger, trigger bar, trigger spring and slide lock together as you install that assembly back into the frame. It's pushed out as you insert the the trigger pin through the frame.More questions: I did not installed the kit provided sear spring. Is it different than the factory sear spring? And what is that yellow thing for? Is that what they call a slave pin in the Apex parts description?
Apex's web site has excellent videos for installing both the action and trigger kits.