1) Baseline Strength
At the end of the day, essentially all athletic performance boils down to strength. Without having the potential to produce force within your musculoskeletal system, no rapid movement is possible. In other words, while one of the larger obsessions in the performance world these days is velocity-based training, where all movements must move fast, "studies continue to support the use of slower exercise, such as resistance training, to increase faster tempo movements like sprinting" (Sparta). And, although it "seemed counter-intuitive at first, [a lack of explosiveness] could be due to lack of ability to create tension (maximal strength) initially, making [slow, maximal exercises] the best way to develop their ability to move quicker." In other words, training slow to a certain extent will allow you to move faster!
However, in no way would I ever suggest that there is no place for moving fast in a defensive regimen. Increasing explosive power and the function of the stretch shortening cycle is vital to becoming a better defender...
2) Power and Reactive Strength (SSC)
Defending, basketball, and athletic performance and general are all twofold: how much force you can produce, and how quickly you can produce it. This is where power comes in.
The definition of power, scientifically, is (force x distance)/time--or, in English, how quickly one can expend force. If someone is able to produce an extreme amount of force but expends it over a long period of time, they will not be nearly as explosive as someone who expends it very quickly. Thus, athletes must train their muscles and central nervous system to produce force quickly.
Related is the stretch shortening cycle (or reactive strength), which is the process in which an "eccentric/concentric muscular contraction produces a more powerful output than purely concentric action" (PUSH). In other words, the quick transition from the absorbing (eccentric) to exploding (concentric) motion creates an explosive function that serves as "the link between traditional strength training and high speed skill" (Sparta).
3) Balance and Stability
With moving fast comes the responsibility of slowing down. If unable to properly decelerate, not only will performance be hindered, but injuries will occur. In fact, deceleration and landing are the primary cause of knee ligament injuries, such as ACL and MCL tears and sprains. Since on-ball defense requires constantly stopping on a dime, stabilization must be a primary focus of any regimen. This is primarily done through eccentric, or slowly lowering, exercises in the weight room (which increase the capacity to absorb force), and sticking exercises such as one leg mini-hurdle hops that train the body to stabilize on landing.
Also, defense is consistently very physical, and to be able to cut off defenders and hold your position inside the arc, you must be able to absorb contact, as you see in the video below.
4) Movement Work
Defensive movement is a skill. While maximal and reactive strength, along with stabilization, are all needed, without the ability to move efficiently, players will never meet their full defensive potential. Thus, players must explosively train their central nervous system--muscle memory--to perform these techniques--specifically phase changes (sprinting to sliding, sliding to backpedaling, etc.), sharp cuts, and the application of the movements that they have been training for (Craig). To do this, cone drills such as shuttles, box drills, and more should be performed in order to increase coordination and proprioception, allowing for more efficient movement. However, they should not be performed for more than 10-15 seconds, or otherwise they become un-explosive conditioning exercises.
However, while training movement in a controlled setting (as stated above in number four) is beneficial to a certain extent and a necessity in most stages of training programs, agility drills in conjunction with reaction is the step that can take many to the next level. With pre-determined exercises, "you learn to anticipate the next move. Thus, agility training can be more effective if the athlete has to respond to a directional order" (Craig). Also, pre-determined movements do not allow for various joint and cutting angles that will occur in unpredictable reaction exercises.
It makes sense if you think about it. When you defend a player on-ball, do you know exactly when they will explode, and to which direction? No! Your brain must read which movement is required, transfer the command to the musculoskeletal system, and then you move. Therefore, why not train this way, reacting to commands or visual stimulants?
Finally, being able to move through a full range of motion in all joints greatly benefits a defender. Firstly, defenders are able to get low and widen their base, increasing stability, as well as transitioning their body through slides, sprints, and backpedals, by mobility in their hip joints. It allows defenders to safely move through the ranges of motion that their task demands, and thus reduces the risk of injury.
One muscle that must remain loose is the gluteus medius, or the hip abductor.
Debunking Popular False Beliefs
Contrary to popular belief, not ALL strength training for lateral quickness must mimic the movement. This is because, according to Sparta Science, at the end of the day gravity remains the opposition to lateral movement, and because getting low--a vital part of lateral movement--is in the vertical plane. So, while side lunges and other lateral lifts undoubtedly have their place in any training regimen, they should not be performed especially for someone who is training for defense. However, unilateral (one-leg) exercises should never be neglected and performed frequently.
Also, the notion of "quick feet" is actually a flawed theory. Having "quick feet" is actually a result of the athlete producing a significant amount of force into the ground, and thus--through the "reaction force"--the ground propelling them laterally.
Exercises for Each (More on the way)
1) Deadlifts (trap bar, barbell, etc), Squat/Tempo Squats, RDL's, RFESS, Step-Ups, Front/Reverse Lunges, Side Lunges, Sled Pulls/Pushes, etc.
2) Power: Olympic Lifts (hang cleans, hang high pulls, etc), Medicine Ball Alternatives, etc. Reactive Strength: Plyometrics such as broad jumps, depth jumps, split jumps, box jumps, etc.
3) Deceleration: sticking exercises such as single-leg mini hurdle hops with a stick, eccentric/tempo exercises where the lowering portion is 4-5 seconds and the explosion up is quick. Core strength and stability can be improved in a number of ways (more later), and base strength is obtained in number one.
4) Cone drills such as shuttles, box drills, and more on this site:
5) Reaction drills such as tennis ball explosions, mirror drills, or any creative drill that adds visual or auditory cues to agility exercises.
6) Hip mobility: Myofascial release (foam rolling, lacrosse ball release, etc), dynamic and static stretches of the hip flexor muscles, hamstrings, quads, gluteus medius and maximus, and more. Ankle mobility is also required (See triple extension article). Also, fire hydrants, scorpions, and bridges.