Seems to me that the idea behind (underneath?) the Merziac Multiwrench Method is that the wrench on the tool flats will tend to drive the tool into the crankarm, keeping it from moving outward from the force of the extractor's "plunger" part. If the threaded part can't move relative to the threads, it can't strip them.

I have no idea whether or not it actually works, but the theory seems sound. It was actually a bit of a head-slapper for me.

--Shannon