Vertical kick is one of the best ways to get better at kicking in freestyle, backstroke and butterfly. By kicking vertically you force yourself to kick fast and effectively because if you don’t you sink and you can’t breathe. You will need to kick narrow and fast. You will feel how your knees need to be a little bent while going this way and straight while going back. Feel how your ankles point down.
The power of your kick originates from your core and goes all to your quads, hamstrings all the way to your calves. Try rotating 90 degrees using only your kick. There are different levels of difficulty. For example having your hands by the side of your head is easier than having them on a streamline. Also it is more difficult to do butterfly kick, but it is a very good way to work your core. Try it out for 10 seconds with you hands like these on the sides of your head and once you get better at it try going for longer and then with a streamline. A very good workout is to do 10 times 30 seconds of this. alternating between flutter kick and dolphin kick. Once you practice this try doing some underwater kicks of full stroke backstroke or whatever stroke and see the difference.