Blunt force trauma is probably closest to your current method but it's all about location, location, location if you aim for humane. For smaller rodents, a simple flick of the finger does the trick. For larger rodents, something a bit heavier is best. You can hold them by the tail with one hand and use the other to give a swift blow to the head with your object of choice. Keep in mind that the longer you hold a rodent by the tail the more they are going to struggle making it harder for your aim to be true.
Another method of blunt force trauma involves swinging the rodent into a stationary object. A tabletop corner works well and is my recommendation for beginners as it also give a fair chance at cervical dislocation if their aim isn't the best - just shoot for the head and neck region. You grasp the rodent by the base of the tail and give one steady swing into the object. This doesn't require 360 degrees of fast flying rodent. What matters most is your accuracy and the more swinging around you're doing the less likely you're going to get it right.
I suggest following either with cervical dislocation just in case and it only takes an additional second or two. I'll describe this as if you have just hit the rodent with blunt force trauma and are still holding it by the tail. Keep in mind that a dispatched rodent will often kick those back legs around for a short while but this is no time to get timid. You owe it to the critter to make sure the deed is done.
Continue holding the rat by tail and place it on a table top. Put one finger from your free hand on the neck and firmly pull up on the tail while maintaining pressure with that finger. When pulling the tail you want to go up and slightly towards the head. If you simply pull away you won't have any torque and are likely to just strip the skin off the tail. You can use an object instead of your finger but will feel the dislocation better with both hands on the rodent.