what is win 10 technical preview.and what its advantage.is lumia930 does not support it.

I am quoting from the blog article:

"Some context on why we chose these and not higher end phones like the 930/Icon or 1520: We have a feature that will be coming soon called partition stitching which will allow us to adjust the OS partition dynamically to create room for the install process to be able to update the OS in-place. Until this comes in, we needed devices which were configured by mobile operators with sufficiently sized OS partitions to allow the in-place upgrade, and many of the bigger phones have very tight OS partitions.

Note that this doesnt mean that Windows 10 will take more disk space than Windows Phone 8.1, its just a function of the upgrade process at this point. Once the partition stitching feature is completed, many more devices will be supported."

Ref:

Announcing the first build of Windows 10 Technical Preview for Phones by Gabe Aul