As MJ suggests the student is having to make a concious effort to follow your guidance and I would imagine that his capacity has reduced meaning a loss of concentration on other things. Once te new routine becomes more automatic then the capacity will increase and you might even see a better performance when the appropriate technique is used effectively.
We get this a lot with students that have say 150/200 hrs doing something a particular way. When we introduce the correct technique or company method their performance deteriorates as the try to unlearn and relearn something.
Standardisation is the key.