In volleyball, blocking is the first form of defense. Your blockers, where you place them, and their level of play, can determine where you place the defend by them and can determine what kind of defense you might play as well.
A common misconception is that the higher you jump and get over the net, the better block you will put out. But it not so much about jumping above the net as it is more about pushing over the net, into the other player's range.
If you block with your arms straight up into the air, you will be very tight to the net and the ball will more than likely fly off your fingers. If you block too far from the net the ball might come down in between your body and the net instead of on the opponent's side of the court.
When a coach wants to increase their defense, blocking should be addressed and focused on as it is the first form of defense. Here are a few of the best tips, dril
1) Work on Your Core
While blocking is about jumping, it is also about your core strength. You need to have strong shoulders and a strong core in order to continue to block again and again with strength that will force the ball back onto the opponent's side of the court. The Art of Coaching Volleyball has specific volleyball workouts (https://www.theartofcoachingvolleyball.com/10-volleyball-specific-strength-exercises/) that are tailored for volleyball players because of the focus on your core and shoulder strength.
One example of a great exercise is an ab exercise where you sit on a mat and have a plate in your hands. Then you take your feet off the ground and complete a side to side twist, touching the plate to the ground, and then after pushing the plate above your head, powering your shoulders. Focusing on shoulders will definitely help increase your blocking skills, but it is noted that if you're in season focus less on weight and more on movement and then when you're out of season you can increase the weight to gain strength to prevent injuries from stressed and overtired muscles.
2) Mirror Blocking
Mirror blocking is a go-to drill that enables two players to focus on blocking at the same time. Players are located on different sides of the net, the first player performs a blocking movement (it might be a single step-hop, or another form of blocking depending on what your coach has you do) and then the second player mirrors the first player's movement. Practicing this drill enables the partners to react to each other, focusing on the person on the other side of the net (not the volleyball!) while at the same time working on blocking movement and footwork.
3) Lateral Lunge with a Press
The lateral lunge with a press is another workout that will help increase your blocking movements. You have a plate in your hands while you complete a lateral side lunge, and then when you are down in your lunge you push the plate straight forward in front of you and bring it back to your chest before you move outwards out of the lunge. This is a great workout because you develop side to side movements while also working on developing your core muscles as those muscles are taunt and flexed while you're completing the lunges (video can be found on same link as above from The Art of Coaching Volleyball).
4) Timing is Everything!
You can be the best jumper and have the strongest shoulders, but if your timing is off than your blocking will be off too and your defense will suffer because of it. While you work on blocking drills make sure to focus on the timing. The most common mistake is for blockers to watch the ball instead of watching the hitter. Your eyes should move from the pass to the setter, to the set quickly, and then immediately back down to the hitter to watch the hitter's timing and angle of approach.
One great drill can be found here: https://volleyballexpert.com/volleyball-blocking-drills/. It is where a ball comes from the same side as the blocker to a attacker on the other side. The attacker attacks the ball like it is a set and the blocker needs to not watch the ball, but focus on the hitter and the hitter's timing in order to set up the block. The goal of this drill is for the blocker to not watch the ball but to focus on the hitter and the hitter's approach.
5) Master the Footwork
Like learning a hitting approach, once you master the footwork than your mind doesn't need to think about the feet and the footwork, and instead your mind can focus on simply blocking. Once you master the footwork your feet will take you to where you need to go and your eyes will direct you where to set the block up, when to jump up, and where to exactly place your hands.
There's a side step block, a cross over step block, and a run jump block. Depending on your coach and how your coach believes blocking should be done your blocking footwork might look different than someone else's, but generally the footsteps are the same and can be practiced with and without a volleyball. The University of Manitoba has a great form to follow with pictures to learn the best footwork for blocking: https://umanitoba.ca/faculties/kinrec/hlhpri/media/footwork_block.pdf.
Blocking IS the first form of defense and can be a game changer during a match. When you continually shut down a hitter, the opponent's will become stressed, desperate, and more apt to make mistakes in order to avoid your hands.