Removes the smooth mesh created by Proxy > Subdiv Proxy (with Mirror Behavior set to Full) and combines the two halves of the proxy mesh (the original mesh and the mirrored mesh) into a single mesh (similar to using Mesh > Mirror Geometry on the original mesh).
Adjustments that you make to one half of the mesh will no longer be automatically mirrored onto the other half.
After you have created a symmetrical model using Proxy > Subdiv Proxy (with Mirror Behavior set to Full), use Proxy > Remove Subdiv Proxy Mirror to remove the smooth mesh. Then use Proxy > Subdiv Proxy (with Mirror Behavior set to None) to create a smooth mesh that you can animate non-symmetrically.
Proxy > Remove Subdiv Proxy Mirror > 
The direction that the original proxy mesh will be mirrored. (Set Mirror Direction to the same setting you used for Mirror Direction with Proxy > Subdiv Proxy.)