Actually, it sounds like the stretch that Aarjan is describing is different from the sankajo/sankyo stretch. Here's the basic sankyo stretch that's done in most dojo:
I think the stretch that Aarjan is describing can be done by first putting your left hand in front of your face, palm facing toward you. Straighten the left arm out some (not all the way so as to keep from hyperextending the elbow) so that the tips of the left hand are now pointing out, palm up. Grab the fingertips of the left hand with your right hand and gently tug downwards. If you're flexible, you can bend the fingers back towards the elbow. This should stretch the inside of the arm pretty nicely.
I can't say I have a name for the stretch, though. Sorry.