Pulling a roving thru a diz from a drum carded batt can give a great preparation to spin. Here is the roving started. I use one of those plastic marketing credit cards with a hole poked in it as a cheap diz.
This shot shows the diz threaded with the beginning of the batt.
I pull the fiber away from the drum, the diz usually comes with it.
I then push the diz back towards the drum while maintaining tension on the roving.
Repeat all the way across the drum. Sometimes some fiber is left on the drum - it can be lifted up with a utensil to be incorporated into the roving