[EricH]

Originally Posted by MARS_James

This is the rule that makes it illegeal:

G40 A ROBOT may not cause a BOULDER to move from the NEUTRAL ZONE into the opponent’s
COURTYARD unless:
A. the ROBOT contacts the BOULDER within OUTER WORKS, and
B. the ROBOT completes its CROSSING (e.g. doesn’t completely back out of the OUTER
WORKS into the NEUTRAL ZONE)

They then define crossing:
CROSS an act performed by a ROBOT, such that it starts free of contact with an opponent
DEFENSE and completely in the NEUTRAL ZONE, traverses the DEFENSE such that
its BUMPERS go fully between the adjacent SHIELDS/GUARDRAIL, and ends up fully
contained by the opponent’s COURTYARD

If you look at the examples in the blue boxes, particularly example B, there is no requirement for the robot to remain in contact with the Boulder throughout the Crossing.