The CW originally planned to make Arrow episode 2.20 into a back-door pilot for The Flash, bringing back Barry Allen from his two-episode guest spot and showing him after he gains super-speed. The network liked the character so much that they ordered the stand-alone pilot that’s currently in production.
But they’re still establishing background for the Flash spin-off, as two members of the supporting cast are confirmed for Arrow episode 2.19: Danielle Panabaker and Carlos Valdes will appear as Caitlin Snow and Cisco Ramon, scientists based on Killer Frost and Vibe from the comic books. (via FlashFans.org)
Update: THR has more information. Guggenheim explains that bringing in the S.T.A.R. Labs scientists keeps the Flash elements present in Arrow in a much more interesting way than showing Grant Gustin lying in a hospital bed with his eyes closed.
— Arrow – GreenArrowTV (@GreenArrowTV) March 17, 2014