From what I know (not much - still learning!!) you have a review at the end of the 5 years to see if there's anything to hand over, then payments are set for the additional 12 months.
At the end of the 6th year I believe there will be a final check to make sure you've earned nothing extra during that year that might need to hand over.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk
My understanding is the additional 6th year is still under the same conditions as years 1-5 and therefore any additional income needs to be reported in and paid across etc.
Only when you have reached 100% of your debts, IP fees and possible statutory interest will your IVA end any earlier than the 6th year.
Sharing from experiences of dealing with debt
There is a solution for everyone .... Just need to stay positive !