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one extra step is done whenever at LSB $10$ or $01$ is seen.
Booth's Multiplication Algorithm reduces the number of additions and substractions done during the multiplication.
The number of additions done is equals to the number of times +1 occurs in Booth recoded multiplier and number of substraction is equals to number of times -1 occurs in it. So the worst case occurs when pairs of 01s or 10s occur very frequently in the multiplier, as it increases the number of +1 and -1 in the multiplier...