We know that flag byte is added in the beginning and ending of frame but if the data part contains the flag then the receiver wrongly predict that it is the end of frame .
To avoid this In bit stuffing, if a 0 and five consecutive 1 bits are encountered, an extra 0 is added. The extra bit is added after one 0 followed by five 1s regardless of the value of the next bit
Fix Flag is used:- 01111110
So , 01111110 011110111110011111010 01111110 -[ Bold part is flag bit]