The backwards double knees (or whatever you want to call them) are super weird, too. I'm just hoping they look much better than the concept when they get in game.
They have normal knees. What appears to be a second backward knee is actually the heel of their foot and it's called being digitigrade (walking on digits or toes). So they just have really long feet and they basically tippy-toe everywhere. They share this trait with wolves and most other animals in the dog and cat families. It's a really useful quality to have. If you think of the Skar more as a primal animal/insect, they're actually pretty cool.
Maybe helps them jump and pounce?
Exactly. Digitigrade animals are exceptionally good at leaping and ambushing. It actually reminds me of the Zealot from SC2 (http://orig06.deviantart.net/81ab/f/2010/322/1/2/121a7cc18886cd43017785690ea85b5f-d334vz3.jpg) which are extremely fast and agile runners, jumpers, and Warriors. I'd be willing to bet that a Skar would actually be able to outrun any other race. And a Skar running at you full speed would be truly fearsome.