I don't think "hang on payments" is a valid expression in English.

It may mean that the hourly rate will in future cover all other payments formerly made separately in addition to it.

It is also possible, though less likely, that it may mean "hang on" in the colloquial sense of waiting a while longer, so if you have to hang on after normal working hours, you won't get any payment for it other than your normal hourly rate which has been increased to cover it.

I don't like the term but this would seem to mean "delayed payments"

HTH

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Note added at 3 hrs (2006-07-13 21:33:29 GMT)
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Example of use:

Civil law also determines that the interest rate has to be paid for delayed payment of debt, even if the debt itself has no interest, as well as for delayed payment of invoices.

if they are to be included in the hourly rate, they can't be payments already owing.
Hanging on for something is waiting for something as Jack says.

In some contracts certain work or procedures must be followed before other work or procedures can commence.

I do agree with Jack that it is rather colloquial English to find in a contract, unless it is an accepted jaron term in the sector concerned.

An alternative is that the payment is a sort of retainer payment to enable a contractor to "hang on" to staff between phases of a project.