section 15/15 ?

Are you referring to the credit consumer code Sect 15/15
http://www.austlii.edu.au/au/legis/qld/consol_act/ccc176/s15.html

or
ITAA 1997 Section 15/15
Tax profits for speculators
http://www.dealersgroup.com.au/kb/cf37—les-szekely-paper.pdf

or
A ‘Section15-15 Withholding Tax Variation’ is a form that can be lodged with the Australian Taxation Office
http://www.mgtps.com.au/site/DefaultSite/filesystem/documents/s15-15Form20082.pdf
http://www.ato.gov.au/individuals/content.asp?doc=/content/00096541.htm

I will assume you are referring to Section15-15 Withholding Tax Variation.

When you negatively gear a property or a couple of properties you may find that at the end of the tax year you get a large refund say for example a refund of $4000 off the tax you already paid while you worked in a job and earned a wage that had tax taken out of it based on an annual wage of $$$$ amount.
Well the ATO holds onto the $4000 until you put in a tax return any where from 12 months to about 23 months if you are slack and use an registered tax agent. Then they assess they owe you a refund and pay you.
When the commissioner of taxation approves your section 15/15 Withholding Tax Variation form you get taxed less each week so that you can stick an extra  $76 into your loan repayments or it might help you buy another investment property.

However if you stuff it up and get the amount wrong it !