2kSS,
The reason Chris states "locking up at WOT" is because once you lock the converter clutch the input shaft has ZERO give from flywheel torque to driving the trans internals.
With the converter unlocked,
the input shaft has some give on a shift, so it sees the torque load, but it doesn't see as much shock on a shift due to the converter taking some shock.
With alocked up converter the shift, as far as the input shaft is concerned, is like power shifting a manual trans. Instantaneous change of torque load.