IBR and contract work

I am but confused on how this work. If you work full time all year around then you recertify with your taxes.

1. If you lose your job or laid off or quit then you can recertify again

But

If you work contract work that is on W2.

Year 1

Worked February -june only

Year 2

worked Jan -march

Worked may-july

Year 3

Worked jan- August

Year 4

No work

How do you go about recertifying the income? Because each year the taxes are so different. So you might have made a lot of money the year before but now you don't so you would actually pay more than 10%. Do you just recertify more often then? When you find a contract and when it ends?