Sorta... the hammer stays down but the hammer spring stays compressed so you have a long but light pull. There're a few versions but the gist of it is there's a short reset and a long reset. short reset acts like SA but if you let go of the trigger to the second reset the hammer's down (decocked) and it's a long pull (but light since the hammer spring's still compressed). If you have a dud primer, there's still a true DA for a second strike but with full hammer spring weight. sounds goofy, takes a little getting used to, but everyone that's gotten through the adaption period loves it.
Have handled/shot only a handful of sigs (never a legion tho) and yea they have a crisper trigger (less takeup mainly) but my P30 just melts into my hand.
Are you sure your not refering to a Sig with a DAK trigger? The HK I examened the hammer stayed cocked and it was basically like a SA after the first long DA pull/shot. I used a Sig DAK before wirkwise and wgike bit bad that is an aquired taste
This shows what I referred about at about the 8 min mark
https://youtu.be/1KVkWGMkOKE