If you've been playing through Dragon Ball Z: Kakarot since its release last Friday, chances are you've progressed far enough to unlock one of the transformations for the playable characters. For those who have those powerups, you might've even pulled off something called a "Super Finish" or "Flashy Finish," possibly without knowing how you even did it. Intentionally performing one of these movies is pretty simple though, and it's one of the best ways to get an even more cinematic experience out of Kakarot.
To have any hope of pulling off a Super Finish, you'll need to get to the point to where you unlock a transformation. The first character who gains access to those is Goku when he's able to use the Kaioken transformation, but you can use it afterwards with the Super Saiyan ability as well.
Once you have the transformation you want to perform a Super Finish with, you'll need to wear an enemy down to the point that you're pretty sure your next super attack will finish them off. Make sure that you've got your Tension Gauge totally full, then activate whatever transformation you want to use with the finishing move.
After activating the transformation, hold down the button that charges your Ki. Assuming you've got a full Tension Gauge, you'll enter Surge mode as you've probably done multiple times with and without your transformations. With both the transformation and Surge active, let loose a super attack on your enemy.
Assuming everything was done correctly, you'll see a brief animation play out that looks like it was pulled straight from Dragon Ball Z. If you use the Kamehameha, for example, you'll be treated to a close-up on Goku and then a view from space where the Kamehameha leaves the planet's surface in a dazzling display.
It's a fancy way to finish a fight, but other than the cinematic value and a trophy awarded the first time you pull off a Super Finish, there don't appear to be any rewards tied to the move in the form of Z Orbs or other resources. It appears that you can perform these moves whenever you want so long as you have everything in order, but if you don't want to cheapen the effect, consider saving the Super Finishes for the biggest opponents from Dragon Ball Z.