My memory is hazy but we synced the generators on the 727 using phase lights. If you rushed the sync you could get a little shudder in the A/C.
As part of my expansion of CV during the Arab Oil Emargo, where no flying gig was available, I did a stint in a nuclear/fossil power plant. They told a story of a many ton generator being twisted off mounts when somehow put online out of phase. Off topic sorry.