Doesn't it always seem like there are different instructions on how the officers are told to conduct a DUI investigation and what is in the student manual? Sometimes, the instructor manual may have more detail about how the officer is required to perform the test. When that happens, it can be difficult to impeach them. But, with the instruction manual, you may be able to demonstrate that what the officer said they were told is not in the truth, or at least not supported by the evidence. On the other hand, the officer may have been instructed to follow certain techniques and failed to do so.
So look at the instruction manual to see if the officer is doing it right.