The SMA tube has functions similiar to the SMA ribbons and the SMA splint pin. the tube lies in a circular cavity between the two layers of the assembly, which is held together with a 'catch'.
At Af the SMA tube reverts to a pre-trained oval shape, pushing the layers of the assembly apart, forcing the catch to release. The tube could also be pre-trained to bend along its axis for additional force, if required.