I offer the opportunity to ask questions after basics are finished, after basic kicking is finished, after forms are finished etc. I don't allow questions DURING each set because to stop to answer questions would disrupt the flow of practice and bring down the students who are trying to concentrate.
After each set is done, I will simply ask them "Does anyone have any questions?" If it is not a simple question, I might meet with them later to discuss it. After all, I really don't want to be there all night answering questions, and we have other things to get to.