heat and ice.
I thought i put it clear enough...its the heat from the fluid that melts the ice....the de.ice fluid stops it reforming on body parts,wings ect so aircraft has correct airflow over and under wings during take off.....fluid shears off when aircraft takes off............ (basic version) for none de.ice person.
i have watched people de.ice doing a spot here and a spot there on the wings...so not all of the wing has been done...they only spray the ice they see.....so ice must form on the bits they did not spray .....so there is no point in doing five spots on one wing and three on the other.....its got to be symmetrical.