If you want to get better at jumping, you need to strengthen the muscles involved in jumping according to their biomechanical and anatomical function. Secondly, you need to practice jumping. It is that simple. Of course, the strengthening portion is not "simple" - simple to understand, not simple to actually perform.
The skills are very specific. If you want to get better at say punching....them punch. Punching while holding a weight will not help - it is a different movement.
Similarly if you want to get better at swinging a standard bat.....then swing a standard bat...not a heavier one. Sure train to get stronger so you can swing harder....but swinging a heavier bat is a different skill.
Perhaps that is what this study is saying:
Effect of Warm-up With Different Weighted Bats on Normal Baseball Bat Velocity.
Traditionally, baseball players have used a heavy bat for warm-up before competition. Because bat velocity is an essential component to hitting a baseball, and because players warm up differently, there is a need to investigate the best way to maximize post warm-up bat velocity. The purpose of this study was to determine the effects of warm-up with different weighted bats on normal baseball bat velocity. Nineteen recreational male baseball players (age, 24.5 +/- 3.9 years; height, 181.1 +/- 8.4 cm; body mass, 87.9 +/- 18.4 kg) participated in this study. Three different randomized warm-up conditions were completed and analyzed for velocity and for their effect on post warm-up normal baseball bat velocity. Subjects were instructed to perform 5 maximal swings with each of 3 different weighted bats-light (LB = 9.6 oz), normal (NB = 31.5 oz), and heavy (HB = 55.2 oz)-followed by 30-second rest and then 5 swings of the NB. Analysis of variance revealed that warm-up velocity of the LB (63.57 +/- 3.58 mph) was significantly (p < style="font-weight: bold;">it is suggested that when preparing to hit, 5 warm-up swings with either a light or normal bat will allow a player to achieve the greatest velocity of their normal bat.