Just like normal combined pills, Evra works by fooling the body into thinking that Ovulation has already occured. This prevents an egg from ripening and being released.
The combined hormones also have an effect of increasing the thickness of the natural mucus at the neck of the womb, which makes it increasingly difficult for sperm to cross from the vagina into the womb and onwards to meet an egg.
The hormones also have an effect on the quality of the womb lining for the duration of that cycle, making it less likely that an egg can implant there.