The groove ground lengthwise into the underside of the blade, which is what creates the two edges in the first place. Quoted in inches: a smaller radius means a deeper groove and more grip.
A blade does not glide because pressure melts the ice, but on a highly mobile surface layer — a wafer-thin film of water between metal and ice. The hollow decides how you use it: the flat area glides, the two edges bite. At a radius of about one inch the groove is shallow, like a gentle bowl, so the blade runs easily and barely digs in; at 3/8 inch it is tight as a notch — plenty of grip, but noticeably more drag. The typical mistake is assuming more grip is automatically better: for beginners a deep hollow chokes exactly the glide they are trying to learn. What gets overlooked — the ice has an operating point too. Friction is lowest at around minus seven degrees Celsius; near zero the ice softens and the blade digs in. That is why the same skates feel different from rink to rink.