A supreme court decision in the late 1990s indicated that a police officer can search for weapons w/o the previous need to determine probably cause related to an arrest for a misdemeanor or felony. The only cause the officer needs is "personal safety" and this can be applied by the officer before they even question you or applied as a matter of policy. For example, that's why on drunk stops as a matter of policy they can remove you from the car and "pat you down" for weapons before they question you or give you a sobriety test. They used to have to have cause, such as aggressive behaviour or actual evidence of intoxication, such as whiskey breath.
Your sister-in-law is correct you need cause to search someone for theft, you have to have probable cause, but she can pat them down for weapons as a matter of her personal safety. Then she can remove that hard object in their pants to see what it is...low and behold its the swagg.
This isn't law, but court decisions, I've seen or experienced this personally in big midwestern cities to tiny new hampshire towns. The original "law" was not law at all but court decisions, which have changed with the more conservative supreme court.
Your kidding yourself if you think there is a legitimate purpose for the wave other than as a weapon. The inventor certainly creates weapons and markets the wave for that purpose. A one-hand or assisted opener knife would get your knife open quick enough for any emergency. The wave brings your knife out of the pocket, open and ready for use as a weapon...just like a switchblade.
Remember laws change as you move around. I used to live in a place where my balisong was legal, until I went 10 blocks west then it was specifically classified as a weapon and was equivalent to carrying a gun.