As shown in Eq. 2, the neutral pathway of the Heck reaction begins with the oxidative addition of the aryl or alkenyl halide into a coordinatively unsaturated palladium(0) complex (typically bound to two phosphine ligands) to give complex I. Dissociation of a phosphine ligand followed by association of the alkene yields complex II, and migratory insertion of the alkene into the carbon-palladium bond establishes the key carbon-carbon bond. 
Insertion takes place in a suprafacial fashion, but the dihedral angle between the alkene and palladium-carbon bond during insertion can vary from 0° to ~90°. After insertion, β-hydride elimination affords the product and a palladium(II)-hydrido complex IV, which is reduced by base back to palladium(0).