Most hard disks in PCs have an "interleave value" from 1 to 7.  This value 
represents the spacing between groups of data on your hard disk.  These groups 
of data are called sectors.  

Your hard disk system reads or writes data on the disk itself as the disk 
spins.  The disk spins at a constant rate.  It is similar to the way that a 
record player works.  The head moves from the edge to the center, and the disk 
spins at a constant rate.  

Your computer can not always accept (or supply) the data fast enough for the 
disk drive.  In such a case, the hard disk system causes your computer to wait 
until the place for that data comes around again.  This takes a little under 
seventeen thousandths of a second.  

If your computer is not able to accept each sector as it comes by, you will 
notice that the performance of your system is slow.  To improve its 
performance, computer manufacturers shuffle the sectors so that the first one 
that is requested is never located next to the second one, the second one is 
never next to the third one, and so on.  

You could visualize this by cutting a pizza into seven slices.  Using a tag, 
number each one. Now change their positions so that none is next to the piece 
that it originally had as a neighbor.  If you evenly shuffled them, you might 
have the arrangement of "1-5-2-6-3-7-4".  In this case, the interleave factor 
is two, since the distance from piece number 1 to piece number 2 is two.  Right 
after you cut the pizza, when they were numbered "1-2-3-4-5-6-7", the 
interleave was one.  If you arranged the pieces "1-6-4-2-7-5-3", the interleave 
would be three.  

The hard disk interleave is performed just like this, but with seventeen slices 
instead of seven.  As the distance between numbered sectors increases, your 
computer has more time to process the data in each sector.  

The manufacturer finds the point where there is enough time to process each 
sector, but not so much time that the computer is waiting too long for each 
sector to arrive at the read/write head.