The short cytoplasmic tails regulate activation of integrin adhesion receptors via clasping/unclasping of their membrane-proximal helices. Using integrin α_IIbβ₃ as a model, we show that a previously reported activating mutation α_IIb(R⁹⁹⁵D) that perturbs the electrostatic interface in the clasp only partially activates α_IIbβ₃ and that extensive activation of the receptor is achieved by complete deletion of α_IIb CT or triple mutations in α_IIb(V⁹⁹⁰A/F⁹⁹²A/R⁹⁹⁵D) that disrupt both electrostatic and hydrophobic interfaces in the clasp. The results provide quantitative evidence for an equilibrium-based integrin activation process where shifting the equilibrium to the fully activated state requires total unclasping of the cytoplasmic tails. We further demonstrate that while the C-terminal region of the α_IIb tail minimally influences α_IIbβ₃ activation, the C-terminal region of the β₃ tail is critically involved. A disease-causing mutation of S⁷⁵²P in this region, but not S⁷⁵²A, suppressed partial activation induced by R⁹⁹⁵D or the talin head domain but did not affect activation induced by α_IIb truncation. NMR spectroscopy revealed that S⁷⁵²P but not the S⁷⁵²A mutation disrupted a C-terminal helix within the β₃ tail, suggesting that the C-terminal helix may regulate the equilibrium-based clasping/unclasping process. Together, these data provide molecular insights into how distinct regions of the cytoplasmic tails differentially and cooperatively regulate integrin activation.