This video shows the operation of the Thrust Reverser on a Trent 1000 powered Boeing 787.

In the video you can see the blocker doors closing the DDuct (aka the Fan Duct) when the Thrust Reverser section moves rearwards. As it moves rearwards, it opens the sides of the engine, the Fan Air moves out through the sides instead of going out of the back. This directs the thrust of the Fan through the Cascade Vanes on the sides of the Thrust Reverser (seen in the video), the cascade vanes are basically grills that direct the air forwards to aid braking after landing.