Here is a link to the FAQ that is designed to discuss this: viewtopic.php?f=21&t=2407
. We also have more information here: viewtopic.php?f=21&t=3266
. These FAQs are located in the "Presale" section of the Frequently Asked Questions forum.
The accelerator only applies to games which show it as an option on the Order page. Specifically, these will be games based on HL1 and HL2.