What I think the OP was speaking of, is an interpretation of the requirement that the truck not be moving during lift and tilting operations, which hopefully we all teach and do, as it changes the center of gravity while moving (not safe). This has, for a long time, been understood in the UK that [during the -testing- phase of operator training] means to put the parking brake on and truck in neutral while raising and lowering. My understanding/ assumption was that this is because the trainee is not, at that point, considered an authorized operator, and added safety measures are called for, for the safety of both the operator and the instructor.
In my not so humble opinion, as a continued operation, this would be incorrect, as the park brake linkage is not designed for that many operations a day, and would result an an early failure of the linkage.
As far as mrfixit's remark, I am sure he also has an operator that is aware there is NO lateral motion (forward, back, or side shift) and only up and down motion while he is in that basket, and that the operator knows that under no circumstances should he leave the operators position at any time a person is in the basket. AND that if there is any time a hard hat or bump cap is called for, this is it, as the numbers I have seen and my own experiences relating to falling object hitting the operator show that more smaller objects fall through the OHG and hit the operator while using a man up basket than any other time. Most manufacturers suggest always wearing a bump cap or hard hat anytime when operating a truck, if you read the operator's manuals.
I don't think anywhere other than the UK uses the requirement of parking brake to insure no lateral motion while lifting, even in testing, but if there is, I would like to hear about it.
I think your "American site boss" has miss interpreted the British requirements slightly, and maybe should be offered the operators manual of the unit, to see what the manufacturer says is correct operational procedures.