An intrapulpal injection achieves effective anesthesia primarily because of the high pressure applied during the injection, which forces the anesthetic solution directly into the pulp tissue. This pressure temporarily blocks nerve conduction by physically distending the nerve fibers, enhancing the anesthetic effect. The anesthetic solution alone is insufficient; it is the combination of mechanical pressure and the anesthetic agent that ensures rapid and profound anesthesia within the pulp. This technique is especially useful in cases where conventional anesthesia fails.

Reference: Malamed, Clinical Practice of the Dental Hygienist, 13th Edition.