When the reading head moves, the state information must be transferred to the next cell of the tape to be visited using only two internal states in machine B. If the next state in machine A is to be (say) state 17 (according to some arbitrary numbering system) this is transferred in machine B by “bouncing” the reading head back and forth between the old cell and the new one 17 times (actually 18 trips to the new cell and 17 back to the old one).
Sounds weirdly like a description of two-track recording, maybe just because of bouncing
