In 1898 his father painted a picture of their garden - but he left out a large branch from one of the trees because it would upset the balance of the picture. After finishing the work he went into the garden and cut off the offending branch so that there would be no difference between the picture and the reality. In 1908 Ernst began a long standing relationship with the occult. Ernst loined the Dada movement as a progression of Communism, for him Dada was Communism but with individuals who had the right to express themselves externally. Ernst's work was heavily collage based to begin with, it was him who developed the frottage technique - which sounds rude but isn't, honest you've all done it.
Frottage involves placing a piece of paper on a rough surface and rubbing over it with charcoal.
His main contribution was not so specific however. Ernst's work was in tune with nature and the subconscious, he was already experementing with automatic writing and drawing and much of his art drew on the effect of his childhood trauma's on his subconscious. Before 1924 Ernst's work was called Dada, after 1924 it was called Surrealism, but his work never actually changed. It always came from his 'natural' emotions and was always backed up by his political brain. After the Dada implosion of 1924 Ernst followed what was for him, a logical progression and joined the Surrealist group with Andre Breton, where he became one of the movements most prolific artists, as he was for Dada.