To perform two-foot dunks, jumpers bend their knees very deeply and spend a lot more time on the ground loading the jump. This increased time during takeoff is useful because it allows the athlete to transfer force into the ground thereby improving height. Using this approach makes it more difficult to transform speed into jump height making a fast approach far less useful than in one-foot jumping.
We have two diferent vertical jump bands that you can use as a volleyball player. The first bands are the M.V.P. Pro bands that are attached at the heels and a belt around the waist. These vertical jump bands can be worn during practice so everytime you jump your are improving your vertical with the resistance provided by the bands. These bands are best if you are 5' 6" or taller.
A second, more efficient and correct method is to use an infrared laser placed at ground level. When an athlete jumps and breaks the plane of the laser with his/her hand, the height at which this occurs is measured. Devices based on United States Patent 5031903, "A vertical jump testing device comprising a plurality of vertically arranged measuring elements each pivotally mounted..." are also common. These devices are used at the highest levels of collegiate and professional performance testing. They are composed of several (roughly 70) 14-inch prongs placed 0.5 inches apart vertically. An athlete will then leap vertically (no running start or step) and make contact with the retractable prongs to mark their leaping ability. This device is used each year at the NFL scouting combine.