As far as I know standing B/C normals must be shielded high, while crouching ones must be shielded low.
Expected behavior:
http://youtu.be/P64yb8WcxM4Unexpected behavior:
http://youtu.be/ZT5Jj071NwMSo it seems that if you attack into an EX shield while it's already active, your attack gets absorbed in the EX shield even if it is not of the corresponding height. It also works with held shield.
In the video example above, although there are only 8 indicators for EX Shield (for the 8 hits of the plant), actually 12 hits are shielded. The shielding character gains meter for all the shielded hits. Also, the trick only works if you hit in the shield within a given time period, which I presume is equal to the hitstop that occurs on successful shield.