Two radical fragments are created.
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Leads to the final products
The cracking reaction involves the homolitic cleavage of a C-C bond. This reaction needs a catalyst to function with any efficiency. This means that the two electrons are divided evenly between the two atoms. This results in two fragments, each with a lone electron (dangling bond). The two fragments are radicals and are not very stable. To stabilize the pair, a hydrogen homolitically cleaves from a carbon neighboring one of the two carbons used to be bonded together leaves that carbon and migrates over to the other fragment to complete the bonding on that fragment. The two carbons on the fragment from which the hydrogen left now form a double C=C bond and all electrons are paired up again. See the diagram below for a visual representation.
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