Ostinatos play a big role in current film scoring and while the principle of how they work is pretty clear, many learning composers struggle with their use. Due to their repetative character, ostinatos can become quite annoying and start feeling short-breathed.
The problem lies in the fact that our brain is not challenged for very long on simple ostinatos and you need to bring in different elements elsewhere to keep it interesting. Have a look at the ostinato below which while being a perfectly fine ostinato runs dead quite quickly:
An easy way to keep this ostinato more interesting for our brain is to lengthen its perceived structure. Essentially, the ostinato below is the same as above. There is just one difference of having the third triplet group in bar 2 different. However this slight change will imply to our brain that the structure of that ostinato is not 2 quarter notes long as the example above but 8 quarter notes long as the “real” repetiton occurs only after two bars. In general, this will keep our brain more interested in this ostinato as we will have the feeling that this ostinato is just a longer sustaining structure, even though it is almost identical to the one above.