Pesquisa functions similarly to Sonar. Out of the Espada, only Grimmjow Jaegerjaquez, Nnoitra Gilga and Ulquiorra Cifer have been seen using this ability, mainly to determine how powerful their opponent is. During his first, unauthorized, attack on Karakura Town, Grimmjow is seen using this ability, alongside his Fracción.
Pesquisa has shown some variation. Nnoitra's version involves putting his finger onto the ground, and a yellow light connects the finger with the opponent whose reiatsu he wants to gauge. Grimmjow's version involves moving into a concentrated state of mind and it works like sonar, expanding outwards from the user.
Out of all the three Espada shown using this technique, Ulquiorra seems to be the most adept with it. He can determine the opponent's level of reiatsu just by looking at them. But, strangely enough, Hollow Ichigo's Sonído was able to pass through his Pesquisa without him noticing.