most often, if you don't need the key to lock it, and only need to push the U in, then the pawl is spring loaded, and only needs pressure to retract. Which means twisting the cut shackle would do the same.
If you need to have the key inserted to lock it, then the pawls are'nt spring loaded and should lock in place, which means twisting won't work without some serious cheater pipes on it.

Alternate way to test would be to 'lock' it without the U installed, and see if you can push the pawl in with a screwdriver. (same as a twisted shackle would be doing)

Ken.