If the wheel is not turning the cable is not being wound onto the drum which pulls up the plunger which makes the mud come out, When you apply tape you stop and cut it, then if you advance the tape without the wheel turning then there is no mud coming out........you got a dry peice of tape, So some people run the wheel a little to adance the tape and put mud behind it then tape a full seam.
But it does not even matter, i know this and often have no mud under the tape at the start you just get that when you wipe it down.....easy.