Two hydrogen atoms — identical, so resonant. Atom A drops n=2→n=1 and emits a Lyman-α photon; atom B, in the ground state, absorbs that exact photon and climbs n=1→n=2. The excitation crosses from A to B carried by one 10.2 eV photon, and then back. A's emission is B's absorption — the same photon, the same energy, because they are the same atom twice. That is the two-transcriber hand-off with nothing added: the photon is the crossing.
LiteralIdentical atoms are resonant; A's Lyman-α emission is exactly B's absorption energy; excitation transfers A→B by one photon. This is how Lyman-α random-walks out of H gas.
BridgeThe two atoms as the two transcribers; the shared photon as the crossing / hand-off; the ping-pong as the figure-eight.
SpeculativeThe clean A↔B line is idealized — real emission is isotropic, so transfer is a random walk (toggle "Isotropic"). Bohr orbits are a cartoon of the 1s/2p cloud.