Many explanations fall into this category. First, the secondary orbital interactions theory emphasized that electron-donating heteroatom in the α-position could contribute to increased orbital interaction with the substrate, which stabilizes the transition state (TS) and gives greater reactivity.  Second, the electron transfer (ET) mechanism presents that the heteroatom in the α position could stabilize the S2 transition state which has a single electron transfer (free radical) character. Other driving forces including the tighter transition state and higher polarizability of α-nucleophiles, involvement of  intramolecular catalysis also plays a role. Another in silico study did find a correlation between the alpha effect and the so-called deformation energy, which is the electronic energy required to bring the two reactants together in the transition state.