In hard disk drives used in computers the binary information is written and read from the disk using a magnetic head. To prevent contact between the head and the spinning disk a wedge shaped region is created at the inlet of the slide surface of the head, resulting in an aerodynamic pressure build up.

The separation between the head and the hard disk during operation, known as the flying height, is one of the important parameters that controls the performance and durability of a hard drive. In order to increase the recording density it is necessary to minimize the flying height. Ideally, zero spacing is preferred. However, zero spacing or contact recording would lead to higher friction and wear at the head-disk interface, hence degrading the performance of the hard drive.

Present-day hard drives work near the ML-HL transition with a flying height between 5 and 20 nanometers (nm). An ideal head-disk interface would consist of a slider where 99.999% of the weight is supported by the aerodynamic pressure. However, by tolerances at the head-disk elements the control of the air bearing partition is limited.
 

Sliders shown in figure 3 and 4 operate in ML-HL transition regime. The major difference between the sets of sliders is the presence of a third pad at the trailing edge and shortening of the two side rails. Shortening of the side rails helps in reducing the flying height and the addition of third pad decreases the contact area between the slider and the disk.

The sliders shown in figures 2 and 4 have a cavity at the leading edge which creates negative (below atmospheric) pressure in the cavity and which reduces the pitch of the slider. Another advantage of using negative pressure sliders is the uniformity in the flying height from the inner to the outer diameter of the disk.
 

 reference University of California http://talkelab.ucsd.edu/index/disk.html