This is what you'll typically see when reverse thrust is applied, the back half of the engine cover slides back on touchdown as it does there are doors on the inside that block the incoming air and direct it out of the gap created by the engine cover sliding back.
So the more thrust you require to help slow down the more you need to increase power in order to get more air pushing out the gap, thats why the engine noise is loud and sometimes gets louder as more demand for "re-directed' air is needed.
I know some pendant will want to correct me as far as technical terms are applied but as a working stiff that flies airliners for the mere pittance that I recieve, thats the simplest way I can explain what's going on !! HTH