There is plenty of info out there on it, but basically it goes between the upper and lower plenums and increases the volume of air That can be taken into the engine. as we all know more air means more power, but if you go too big the air isn't moving fast enough at low RPMs so a bigger spacer will hurt low end power but gain top end. Motordynes 5/16" spacer is small enough that it actually helps throughout the powerband, so it is generally the one suggested for street cars, while race cars and more agressive street build would use the 1/2".

Hey what's up dude? What exactly does that motordyne spacer do?