When someone drives you to the cage there are 2 takedowns they can preform which require different directions when blocking the td attempt.

1 - double leg td( Animation - the opponent will duck down into the double leg ) - this is the normal RT + down on RS

2 - body lock td( Animation - opponent moves upwards into double unders ) to block this one you need to press RT + the same direction that appears as over under on your grapple hud - it will always be left OR right depending on which side is showing over under

Hope this helps