SPR stands for Spacer Pad Response. It is a bearing and response system produced by Buzz-On, intended to allow easy modification of yo-yos. The SPR is, essentially, oversized bearing spacers. The bearing is held between the two spacers by a small bearing seat. The rest of large brass spacer is smooth and flat, allowing a yo-yoer to place any of a variety of friction stickers on it.
While SPR kits were originally produced specifically for Buzz-On products, they found widespread use in other brands' yo-yos for quick and inexpensive bearing and response pad modification.
The SPR originally was introduced in only one size, the large C bearing, but it now comes in three different sizes: large, medium, and small. SPR kits can easily be placed in most yo-yos that use bearing spacers to allow the use of a different response option or a different sized bearing than the yo-yo originally had. Buzz-On currently produces most of their own line of yo-yos with SPR spacers recessed into the sides of the yo-yo.