The best way to learn the Rally Object model is to use the WsapiReference App which can be found here.
Here is what it would look like if you were looking at the Iteration Cumulative Flow Data object.

The best way to think of the Iteration Cumulative Flow Data objects is as a snapshot of a specific Iteration on a specific day. You can find out how many hours of work are left in that iteration and report that way.
If you wanted to look at the velocity for a specific iteration you could use the AcceptedDate on HierarchicalRequirement (what the wsapi calls User Stories). If you queried the list of all stories in that iteration and bucketed them by some timebox(day,week,month) you could report on the velocity of a specific iteration.