The following reaction mechanism incomposes 4 steps. They are the following:

Oxidative Addition- Palladium forms a metal complex between the DMSO and the corresponding carbon.

Addition of Carbon Monoxide- Carbon Monoxide attacks the carbon which has formed a metal complex with the palladium.

Transmetalation- Tin forms a metal complex with DMSO and breaks the metal complex with its carbon while Palladium forms a metal complex with the same carbon and relinquishes its metal complex with DMSO.

Reductive Elimination- The palladium relinquishes its metal complexes with both carbons, causing a bond to form between the carbons.

The mechanism is outlined in the following flash animation: