The Biometric Pressure Spell is a spell that enable 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 it's current location in New York City's Chinatown.
The sorcerer rises 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 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 the said item located. How the said dark cloud manifests on the said area was due to biometric pressure displaces the atmosphere around it. If Grimhold indeed located on the said area, the biometric pressure on the said area would turned the water vapour around it into the special dark cloud that manifests lightning. More lightning manifests on the dark cloud, it means the Grimhold indeed located there.
In order to have this spell works properly, the user must use this spell on the high place.