No Delay means that transactions execute as fast as possible. As soon as one transaction is processed, the next transaction is issued against the server. In the case of a mixed workload test, each virtual user issues transactions as fast as possible.
The No Delay option is used when the goal of the test is to stress the system to its limits, without concern for accurately simulating users. With No Delay specified, a relatively low number of users can stress the system to its limits. However, there is no easy way to correlate N virtual users running with no delay to some number of real users.