I am a certified MEB Instructor. Which is Monadnock Expandable Baton. Monadnock is the original marketer of PR-24 (sidehandle batons), they also market an expandable simular to the ASP, called the Auto-lock. It has a lock mechanism on the end cap used for collapsing the baton, instead of having to strike it on a surface for closing. There are several replacement tips for the various expandables, used to lessen the damage that can be caused by the steel tip. All straight baton training is applicable to expandables, Monadnock mandates a 4 hour class for certification. The advantage of expandables is their portability, when exiting your cruiser in a hurry it won't be left behind. ALL standard batons are primarily used as defensive purposes. Head strikes are your last resort to fend off a deadly attack, they can and do cause severe trauma. Different jurisdictions have differing regulations for civilian possession and use check your local laws. Training is available through several sources, check with local contract security agencies in your area or check the ASP or Monadnock websites. Training usually runs $150-200 for the certification. If you intend to use it for civilian defensive purposes, you should really get the training for liability purposes. If you are certified by a Monadnock instructor for use of their device, Monadnock has legal councilors who can provide assistance in the event you properly use the device for it's intended purpose and face civil litigation. RedMan is the way to go for law enforcement, because it gives you the opputunity to combine multiple defensive techniques. But, it is very intense, and unless your serious about learning how to use non-firearms techniques, don't try it. I don't know if this is offered to non-LE anyway.
Hope this helps.