I do the controlled collapse thing, knock in the centre of the 4 flats to form gentle valleys, (on inch square i do about 6-8mm) then heat and twist. (carefully). same for bending.
I have seen the the inserted bar thing work, and tried it once, i welded a piece of square bar on the end for a non collapsing grip, and cut it of when finnished.
Tube is a very swedgable medium, with very pretty results. just careful on the cooling, ...that you dont have it aimed your way for a jet of steam.