Reed spacers are used to help put an emphasis on low RPM power on a 2-stroke, at the cost of top end output. I'm not an expert in this area, but I do know that an installed reed spacer lengthens the intake tract (between the carbuertor and crankcase) and, I think, reduces the velocity of the incoming fuel/air charge. The engine is better able to make use of the slower-moving intake charge created by a reed spacer at lower RPM's, so it produces better power down low. However, a reed spacer has a sort of 'choking' effect on the engine as it is run closer and closer to WOT, so some power is lost up top.
Unlike boost bottles and the infamous 'ScaryFast PowerNOW', reed spacers DO work. At least to some extent. So does using lower gearing. I don't personally run a reed spacer, since I don't like wrestling my Blaster's clutch on the trails, but you may see some gains with a spacer if you like riding where you need extra low-end torque. My .02.