You use a 1/8" Allen key to turn this bolt to ram the block, locking the string to the saddle.
We searched the Kahler warehouse and found a very limited supply of these! Note that this supply is a version that doesn't have a slot on the head but is otherwise exactly the same (new picture coming soon). If these sell well Kahler will consider making a whole new batch so we can keep the Steeler going for as long as possible. Tell your friends!