The JENZ Precision Machine Heads are specifically designed to anchor the bass string at the lowest point on the machine head post. This reportedly improves the stability of the headstock and eliminates headstock bowing. Designer James Donahue has achieved this by adding threads to the machine head posts, which channel the strings in an orderly fashion up the post while maintaining an anchor point at the base of the post.
The machine heads literally dropped into place on my MIM Fender Precision bass. The pre-existing screw holes were perfectly aligned with the mounting holes on the JENZ machine heads. In minutes I had the new machine heads installed and ready for action.
The first thing I noticed was that the threaded post truly does insure that the string stays anchored to the base of the post. After replacing the G string machine head and reattaching the string, I compared the location of the G string on the JENZ machine head to the D string on the stock Fender machine head. The G string on the JENZ machine head is adjacent to the head stock, while the D string on the stock Fender machine head post is elevated well above the head stock. See photo for more detail.
The JENZ website contains a considerable amount of detail documenting how this design relieves stress from the neck and headstock. If you are interested you can review this data HERE
The manufacturer reports that once the JENZ machine heads are in use that the head stock tree string will no longer be needed. After replacing the machine heads on my bass, I removed the tree string and the strings did stay in place, but I did notice that the D string emitted a prominent overtone that was not there when the string tree was in place and that the G string rattled when forcefully plucked. So, at least in my particular situation, I felt that the string tree was still necessary to have the bass sound and play normally.
The threaded design of the JENZ Precision Machine Heads truly do place the string at the very base of the machine head post. These tuners are well made, attractive and reliably stay in tune. They are priced at $65.00 for a set of four and $78.00 for a set of five. For more information please click on this link for the Jenz Precision Machine website.