The expandable baton is a relatively new development for regular on-duty carry. There are a number of these on the market, ranging from an extended length of 18" to about 28". They are usually metal, and may be stainless steel or black. The tip is usually rubber, polymer, or metal.
They are often referred to as ASP's, since ASP is one of the top brands of expandable batons.
The conventional type is a friction-lock design. These are employed by holding them at the handle and flipping the expandable sections open with a flip of the wrist. The baton sections lock open via friction against each other.
After the baton is employed, it is closed by hitting the tip of the expandable sections on a hard, solid surface. This pushes the expandable sections back into the handle.
An even more recent development is the AUTOLOCK Baton, again manufactured by Monadnock. Instead of a friction-lock mechanism, it uses a ball-bearing locking system. It can be fully extended by either a flip of the wrist, or by simply pulling the expandable sections out until they click into their locked-open position. A button in the endcap of the baton allows it to be closed very easily by hand, rather than by hitting it end-on on a hard surface. Opening the AUTOLOCK baton by hand offers a tactical advantage in that it is quieter than flipping it open.
information courtesy of http://www.commtechreview.net