The chuck is a tooling inside the seamer which holds the end as the can is raised into it and the rolls circle around the can to close the seam.
The chuck is synchronized with the lifter plate and moves together with it. The chuck is held in place and typically does not rotate like the rolls. At the bottom of the chuck there is a small pin that allows the chuck to push out the closed can once the seam has been completely closed and the seamer has rotated into the position where the can will leave the seamer.
The configuration of the roll in a typical seamer head is one chuck (holding the end), a lifter plate (lifting the can and its contents into position) and two rolls. However, there are seamers with more rolls (e.g., 4 rolls per head).
A chuck typically has one or two very specific angles and a radius (the “chuck lip”), which ensures a strong bond between it and the end (lid) during the formation of the seam.
The chuck in the images is at the center of the seamer head.