It is certainly a nonsense that during normal operation, unloaded, a slug can form in a trap ready to be suddenly transported on the instance of completing its formation as US Iceman claims.
This is too easy....

If the system capacity is reduced, the gas velocity through the evaporator and suction line is decreased. When this occurs the return of oil is minimized as there is insufficient velocity to move the oil. At this time the only thing that can move oil is gravity or miscibility of the oil/gas interface. If the flow is upwards gravity works against you.

Eventually, if the system capacity stays reduced long enough the oil begins to pool in the trap (if provided). As the oil pool grows the trap is sealed. As soon as the pressure builds up sufficiently to move the oil slug it flows upward. The pressure buildup can occur as a pressure differential consisting of two things: suction pressure drops due to compressor loading up or the evaporator pressure raises just enough to blow the trap out.

Peter and I are both instructors too and I have a lot of faith in Peters ability and practical experience. We have both worked on equipment and taught others how to and we are still in the same business and still mods here.

I think its safe to say you have two options: believe everything the guest instructor says or keep an open mind and do some research yourself.