Watch a Turing machine compute. The head reads the highlighted cell, writes a new symbol, and shuffles left or right according to its transition function.
Machine
Decides the classic non-regular language of equal numbers of 0s followed by 1s. It crosses off one 0 (→x) and one matching 1 (→y) on each pass.
stateq0· steps 0
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Try this
Run the { 0ⁿ1ⁿ } decider on 000111 and step slowly: you can see it cross off a 0 (→x) and its matching 1 (→y) on each sweep across the tape.