Muṣāfaḥah is a numerical correspondence between one's chain and the isnād position of a compiler's student. The imagery suggests being at the level of someone who could have met that compiler, hence the term “handshake.”
Example: If a compiler's pupil has ten links to the Prophet ﷺ, and a later transmitter has an independent ten-link route to the same endpoint, a form of numerical muṣāfaḥah with that pupil is obtained.
Difference: Musāwāh compares one with the compiler himself; muṣāfaḥah compares with the compiler's student. Neither implies a literal meeting or establishes soundness of the route by itself.