When atoms / groups having lone pair of electrons are bonded by a single bond to the conjugated system, they displace electrons towards multiple bonds.
When electronegative atoms or group of atoms bond themselves to the conjugated systems by multiple bonding they pull electrons from the multiple bond.
This conjugation between electrons of single (H-C) bond with multiple bonds is called hyperconjugation. This occurs when the sigma (s) electrons of the H-C bond that is attached to an unsaturated system, such as double bond or a benzene ring, enter into conjugation with the unsaturated system.