The Biometric Pressure Spell is a spell that enables conjurers to track the Grimhold and, presumably, other magical items.
Balthazar Blake is the only known sorcerer who used this spell to track the Grimhold to its current location in New York City's Chinatown.
A sorcerer performs this sorcerous technique by raising their hands around shoulder high, and then move their fingers while concentrating with their magic. Magic within the caster's body would affect the biometric pressure on the air, and then displaces the atmosphere around the user to locate the Grimhold like a radar (which could be, presumably, as wide as New York City).
If an area where the Grimhold located detected by this spell, a special dark cloud would manifests above the area where said item is located. How the said dark cloud manifests over the said area is due to how biometric pressure displaces the atmosphere around it. If the Grimhold is indeed located in said area, the biometric pressure over said area would turn the water vapor around it into the special dark cloud that manifests lightning. More lightning manifests on in the dark cloud, it means the Grimhold is indeed located there.
In order to have this magical ability works properly, the user must conjure this spell from a high place.