Roche lobe
The donor star will start mass transfer when it overfills its Roche lobe and as a consequence the orbit will shrink further causing it to overflow the Roche lobe even more, which accelerates the mass transfer, causing the orbit to shrink even faster and the donor to expand more. This leads to the run-away process of dynamically unstable mass transfer. In some case the receiving star is unable to accept all material, which leads to the formation of a common envelope engulfing the companion star.