It struck me the other day, being a follower of the "Aiki" or "Internal Power" practices and also someone who trained initially at a dojo where we practiced unbendable arm at first, that Aiki/IP could be described as "unbendable body" -- an extension of the basic practice. The same concept applies, in that the aim is to use the strength of internal connections, rather than muscular power. As much as it may be considered a party trick, when extended to the whole body while doing waza, it definitely works.
I have been on these forums for a while and to my recollection it is quite rare for followers of the "Aiki" or "Internal Power" to agree on anything that in some way pins down their practice. On the discussions I followed, I have never seen Aiki/IP described as "unbendable body".
Alternatively there is Demetrio's video in post #4.