Doing a frequency analysis (FA) is nothing else but counting how many substrings of each possible pattern appear in the complete digit sequence. For example: To analyze the substrings of length L=2 we read the digit sequence in groups of digit pairs collecting every number into it's "home" box:
14 -15 - 92 - 65 - 35 - 89 - 79 -32 - 38 - ...
In the end we will know how many "14", "15", "92" etc. were found.
This is the base for calculating the Chi2-value for the measured sequence to judge about Pi being random or not.
If Pi is RANDOM, each number "XY" should have an equal probability to appear.