Do these on a stationary trainer or outside on a slight downhill. Use light to moderate resistance. Gradually increase your rpm’s until you begin to bounce on the saddle. Back off to the point where you smooth out and then hold that cadence for about 20 seconds. Gradually bring your rpm’s back down to about 90. You can do leg speed drills throughout long rides, or as a transition from warming up to high intensity training.
The focus should be on keeping the leg muscles relaxed and the spin smooth. Think about relaxing you toes and ankles and don’t force the movement. Try and release the tension in your legs and let the energy flow. You’re looking for a smooth, fast, yet effortless motion.
The objective is to teach the muscles in the legs to contract and relax in harmony, or synchronously. If the muscles are fighting each other they will create a resistance to the pedaling movement. This resistance will consume, or waste energy and inhibit your performance.
If you do these on an indoor trainer you should be listening to the whirling, or whooshing, sound that the tire makes on the roller. If the sound is whoosh-whoosh-whoosh then you’re applying power on the down stroke only. Try and get the sound to be continuous/constant. That means that you’re transferring power from on pedal to the other smoothly, and pedaling in one single motion rather than two independent circles.
This is very important for off road cycling. When you’re climbing on loose and/or steep terrain, that whoosh-whoosh will become slip-slip as your rear tire loses traction. Spinning your tires was cool in high school, but it’s a waste of energy in cycling.
These are not intervals. Heart rate and effort levels should be in a comfortable zone. What you’re doing is developing the neurological pathways responsible for instructing the muscles to fire at the appropriate times. As you practice this you will be developing muscle memory. Eventually, it will happen automatically, but like any other skill it takes practice.