Think I get it.
So say Sammael gets a wound. He has 3+ SV. However If he just turbo boosted (+4) and has the dark shroud (stealth, which adds 1 point more to cover) so that would be +3. So I would statistically better off using cover save.
However If I didn't have the Shroud (which would make it +4). I would be better taking my invulnerable save?.
Remember that Sammael's jink save is 4+ when moving and 3+ when going flat out because he has the skilled rider USR.