Not so. The 3/4" rule applies to cut on contact broad heads, and open mechanicals. Many mechanical broadheads are less than 3/4" closed in flight, but open on impact to 1 1/2" to 2" widths. Its the open cutting width that counts against the 3/4" ruleThey must still be 3/4" closed.
If that's the case than 50% of NJ deer hunters are in violation of the law: As for my personal experience, I have been stopped and inspected 3 times while going to and from the stand in the last 5 years. I have, and continue to hunt with both Muzzy 3 blade fixed broadheads, and Vortex mechanicals. While the 3 blades fixed are 1 inch, the vortex are only 5/8" closed-I cary both(vortex for 10+ yard shots and muzzy's for directly under the stand shots) All three CO's passed me on the mechanicals as being perfectly legal despite the fact that they are less than 3/4" closed. (I did not specifically ask them the question-but would assume if I was in violation they would have said something)My interpretation of this would be that the broadhead when closed must be 3/4".
I've never had a mechanical fail to open on impact with a deer. Once in ablue moon, they open in flight after striking an un-seen twig, but never have they "just opened" in flight, and they have never "failed to open" on impact with a deer(or any other creature for that matter. The only shot I do not like with a mechanical is the straight down shot-for some reason, these just do not seem to do the damage I want-way to long and difficult for recovery purposes. That's why I keep two fixed blade tipped arrows in my quiver w 3 mechanicals. I switch to the fixed blade when I get that right under the stand shot opportunitymay or may not open