The phrase "special grooves" refers to unique or specific patterns or cuts in a surface. It can describe the particular design or arrangement of grooves that serve a specific purpose or function. Full definition
There are special grooves on the shoulder they put there to let you know if you're getting off the road. (colorandgrace.com)
A cam slider with special grooves carved into it spins over the adder's pins, reading the pins and levers and ticking the levers back and forth with each revolution until it reaches the desired number and «overflows.» (discovermagazine.com)
The soft natural - feeling nipple also features special grooves meant to help the baby latch onto the teat easily and comfortably. (fussy.org)