Smartreflex

The official explanation

 * "SmartReflex is an advanced, multi-level and multi-parameter power management approach that can be found at some TI DSPs of the OMAP35x series."

TL;DR. SmartReflex is a nifty way of scaling the voltage of the processor in a way similar to frequency scaling. That is, voltage is increased as it is required. Conversely it is decreased as it is not required. Most users report that this has lead to an increase of around 1/3 battery life. (DISCLAIMER:1/3 battery life is dependent on a particular users set up and style of use.)

Stock Nokia kernel
Smart reflex has been considered broken for a long time. Using the stock Nokia kernel it can only be activated up to 805MHz at most. The Stock Nokia kernel implementation of SmartReflex is to be considered broken and its use is to be avoided.

Kernel Power
Since kernel power 50 (KP50), despite some disagreements amongst some of the more senior Maemo developers, SmartReflex can now be considered stable up 900MHz.

So how do I enable it?
Assuming you are using KP50 there are several ways you can enable SR.

list methods here!

Caveats!

 * I know I just said SR is stable up to 900MHz however some users have reported that on their devices SR is only stable up to lower frequencies (720-805MHz for example). For more example on tuning SR see the KP50 article.