Sol.When a carbon attached to at least one hydrogen is attached to an unsaturated atom or one with unpaired atom or one with unpaired orbital of electrons, also involved in delocalisation. This type of resonance is called Hyper conjugation e.g.

The effect of electron displacement due to this type of resonance is called hyper conjugative effect.

There are two theories about it

(i) Resonance theory (ii) M.O. theory

(i) Resonance theory : - Hyperconjugation effect is a σ - π resonance as following

(ii) M.O. theory : According to this theory in hyperconjugation, unsaturated orbital (may be π orbital or unhybridised unpaired orbital or vacant pure p orbital) how ever, can overlap to a certain extent with adjacent orbital to form extended orbitals