How to enable Nvidia Optimus in Asus Notebook(Gt520m)?

I have recently purchased Asus K43sj. It has NVIDIA GeForce GT 520M 1GB DDR3.Problem is that it does not have optimus. Battery life is 4+ hours while surfing. But if I could enable Optimus I could squeeze more battery life. The graphics card itself has Optimus because it is mentioned on Nvidia site and other notebooks having gt520m also have it. Even Asus k53sc(15" model of my laptop) has Optimus.

So Asus has disabled Optimus on this model for some reason. I have searched BIOS settings but there is no option to enable Optimus. Please tell me how to enable Optimus ?

The Asus K43SJ does not use the integrated Intel HD graphics (whether it's on the CPU or not), so you're not going to be able to use Optimus.

I came to this conclusion by the fact that the Intel Graphics are mentioned nowhere on the support page for that model, and the fact that they ONLY offer the nVidia driver (where as the K53SC does provide both the Intel and nVidia drivers).

The technology is there in the nVidia card, the CPU and such, but since this was produced right in the time-frame that Optimus was starting to be implemented, I think you just got a model series (the K43) that Asus put out before they had Optimus ready (I don't believe it was introduced until the K53 series).