By design, Optimus is supposed to start out in (low power) standard graphics chip mode, then switch to the higher power dedicated graphics when called for. But there have been many reports of this system not actually working in reality - people running test programs have shown that even at the highest game settings the graphics never switch.
This could be a problem with the technology, but more likely it is a problem with the game's programming. Without a call from the game and/or OS the system does not know to switch.
The workaround is to bypass the automatic switching and run on dedicated graphics exclusively. But the method to do so involves running a hacked BIOS. This is a procedure that I would never recommend to anyone other than an advanced user. (This assumes that you cannot switch over easily in BIOS as brreaker posted above.)
This epic thread: Acer Laptop: Switchable Graphics using i7 & AMD Radeon
details the procedure. These guys worked out the solution long ago and it remains one of the only solutions I have seen on any forum that actually works.