If anyone was in charge of a large rig -- the gentlemanly thing to do would be to shut down right before a difficulty jump -- and then switch on again. This would maximize profits both for him and the whole network, but I really doubt anyone is pulling such stunts.
That's not how difficulty works. The size of the adjustment is based on the speed with which the previous 2016 blocks have been mined. If that was 10% too fast (so 6.6 blocks per hour instead of 6), difficulty will go up by 10%, etc...
Turning hardware off right before a difficulty jump only has a tiny effect on the size of the adjustment. With the current constatly rising hashrate, the hashrate at the start of each difficulty period is lower than at the end of it, so the difficulty adjustment does not fully compensate for all the new hashing power yet, which is why just after a difficulty adjustment, you'll often see estimates for the new difficulty already being up to 20% or more.