You get to keep up to 6 months equivalent of pay, while you are looking for a new job, to carry on living and paying the IVA. The rest goes to creditors up to the limit of full original debt, plus fees and possible statutory interest. Of the 6 months you get to keep -- if, say, you find new employment within 3 months, you have to give the other three month's worth back to creditors!
This is all assuming that you have the usual clauses in your arrangement.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk