Video explanation
Two potential wells appear as depressions. Growing the donor sends matter across the saddle toward its companion and into a disk.
Roche lobes
In a co-rotating binary frame, critical equipotential surfaces surround the stars as Roche lobes.
Transfer through L1
When a star fills its lobe, matter can stream near the inner Lagrange point toward the companion.
Angular momentum shapes flow
Transferred matter carries angular momentum and may form an accretion disk. The depression represents potential, not a physical funnel.