Hyperthreading is a BIOS level setting. Hyperthreading technology enables multithreaded software applications to execute threads in parallel. To improve performance in the past, threading was enabled in the software by splitting instructions into multiple streams so that multiple processors could act on them.
Hyperthreading technology is a form of simultaneous multithreading technology where multiple threads of software applications can be run concurrently on one processor. This technology essentially fools the operating system into thinking it's hooked up to two processors, allowing two threads to be run in parallel, both on separate "logical" processors within the same physical processor. The operating system sees double through a mix of shared, replicated, and partitioned chip resources, such as registers, math units, and cache memory.
This technology is largely invisible to the platform. In fact, many applications are already multithreaded and will automatically benefit from this technology.
Why is hyperthreading technology interesting in the context of the EPM solution? Hyperthreading is really advantageous for applications that are multiprocessor aware, and generally it is good to turn it on for IIS and SQL servers. The only time when you may consider disabling hyperthreading is when you have separate servers running Views Processing or Session Manager services. The reason is that both of these services can use only a single CPU and therefore cannot take any advantage of the hyperthreading feature. For all other scenarios leave hyperthreading enabled.
If you want to read more about the hyperthreading technology, explore the following resources:
Microsoft Windows-Based Servers and Intel Hyperthreading Technology: http://intel.com/business/bss/products/hyerthreading/server/demo/index.htm
The Register: http://www.theregister.co.uk/2002/06/18/what_the_hell_is_hyperthreading