I’ve never heard of this procedure before but it’s been described recently on the MGOC forum as a way to remove any remnants of air after bleeding the system. If you suspect that the clutch is not operating or disengaging properly then there maybe some air in the system.
The idea is that you leave the clutch pedal fully depressed overnight then gently release it in the morning.
Any air in the system when under pressure will form bubbles that will slowly rise to the top of the loop in the rubber pipe over several hours. When the clutch pedal is gently released the air in the loop is moved up to the master cylinder and released back into the reservoir.
Those bubbles have quite a long way to go! Think i might do this twice one after the other.
I guess it also tests the integrity of the seals in the master and slave cylinder.