The sport of mma is in a perpetual and exponential state of evolution and we are constantly seeing new techniques in grappling and stand-up. I would like to get some opinions on what you think is an effective but under utilized technique in professional mixed martial arts.
In my opinion back fists are very effective and used rarely. In respect to grappling, we are seeing some major innovations in leg locks and this will show up very soon in the cage.
Sidestep.
The glassed bottom boat 
Leg locks are the future..sad but true
Knees to the body on downed opponents. Not used nearly enough and are incredibly devastating.
TKD LOL - Leg locks are the future..sad but trueWhy sad?
reverse triangle on a sprawl
knees to the body of a downed opponent
Von flue choke 
Akido
Karo kimura takedown defense
CroCop spinning sprawl
Yves Edwards suplex/choke slam to side control
Head scissors
The toehold by far
ranier wolfcastle -I hit one of these the other day at bjj. Had trouble finishing it but it's a great position to rain hammerfists on your opponent from too.
reverse triangle on a sprawl
Vomit inducing farts.
Jab
Serious 20 comments and no mention of body shots ? Bros body shots are the most under utilised weapon in martial arts .
Conor mc Gregor is a master of them also nick Diaz throws some wicked body shots
Would like to see more spearing elbows closing the distance, elbows checking kicks to the midsection and duke rufus style kick to the rear leg. Dunno enough BJJ to suggest underutilised moves in that regard, I like the guilotine when someone's shooting though..but that's become a recent trend.
The mount seems to have died out kind of.
That's off the top of my head.
^agreed on bodyshots, but there are a lot of "liver hunters" out there
OmgItsTheWams - Jab
jabs and elbows would be my guess