In the recent article published in 2021, Hansen and Vermeeren proposed the two requirements for an α-nucleophile to present the α-effect. The two proposed requirements were (1) minimization of steric Pauli repulsion via small HOMO lobes on the nucleophilic center and (2) small HOMO-LUMO orbital energy gap that ensures orbital overlap with the substrate. It was proposed that both of these two requirements should be fulfilled to have an α-effect, otherwise, the nucleophiles would show no or inverse α-effect (Figure 2). In this recent work, the six normal nucleophiles (HO, CHO, HN, CHHN, CHS, HS) followed the Brønsted-type correlation. α-nucleophiles with O, HN, and S in the α position were classified into three groups according to their degree and pattern of deviation from the Brønsted-type correlation in S2 reactions with the substrate, ethyl chloride (CHCl) (Figure 3). 
First, the α-nucleophiles with downward deviation, in other words, higher reactivity shown considering the basicity or lower basicity given the reactivity, were grouped as nucleophiles showing α-effect. The second group had nucleophiles with small or no deviation from the line plotted by six normal nucleophiles. Lastly, the third group had nucleophiles showing inverse α-effect, meaning that they are above the plotted line or have less reactivity considering their high basicity. Relative density functional theory, activation strain model, energy decomposition analysis, and Kohn-Sham molecular orbital analysis the three groups had a distinction in HOMO lobes and HOMO-LUMO gaps.
To elaborate on the first requirement, the electronegative heteroatom reduces the electron density of the atom that attacks the nucleophile making the HOMO lobe smaller. This minimizes the Pauli repulsion between the substrate and the nucleophile. Nonetheless, these small HOMO lobes don't mean less orbital interaction than the parent normal nucleophile. This is because α-nucleophiles showing the α-effect have smaller HOMO(nucleophile)-LUMO(substrate) gap, in other words, high HOMO energy level that allows more orbital interaction.
Examples of α-nucleophiles with α-effects are shown in Figure 4. The α-nucleophiles have smaller HOMO lobes than the parent normal nucleophile. 
Examples of α-nucleophiles with α-effect and inverse α-effect are shown in Figure 5.