That's THE thing about the 'free wheel unit check'
with intention to subsequently re-apply power when the autorotation exercise was completed. When the student pilot tried to re-apply power, the helicopter's transmission clutch did not engage, and the rotor speed decreased. The instructor took control and landed the helicopter. After the helicopter landed, the helicopter's clutch re-engaged and the main rotor mast severed just below the rotor head.
It's not that 'needles don't split' - that they 'do rejoin' that we're checking, not that most see it that way.
The consquence of sudden re-engagement can be catastrophic as demoed here. (Another un-factored risk in Twin engined helicopters, (Ian Southridge AS355))