For many years, yo-yos came in two varieties: those made for looping tricks and those made for strick tricks. These two designs have always meant that you needed two different yo-yos. Duncan solved this issue in 2012 with an innovative design that was also very affordable. The plastic Pro Z, part of Duncan's "Hard Core" series, is two yo-yos in one. By adding the yo-yo's enclosed two MOD Spacers (the yellow, dish-shaped parts seen inside the package) and longer axle rod, you can widen the string's catch zone to make it easier to accomplish string tricks. When you remove the MOD Spacers and swap out the long axle rod for the short version you now have a yo-yo great for looping tricks.
The MOD Spacers use Duncan's Friction Stickers for the response. When used without the MOD Spacers, the yo-yo uses the traditional starburst configuration that is molded into each halve. Uses a size A ball bearing. With the MOD spacers and long axle rod, the specifications of the yo-yo change to 59 grams weight and an overall width of 40 mm.
Duncan sold separately three different configurations of MOD Spacer kits, depending on your playing style and the Duncan yo-yo model used.
Duncan also released the "Eagle 1" which is a Pro Z body without the MOD Spacers, and a plastic transaxle instead of the size A ball bearing.