CF = bit(dest, source) ; extract & flip bit numbered by source from dest bit(dest,source) = ! bit(dest,source)
The btc instruction uses the source value as the bit number of the destination to test and complement. The destination can be a 64, 32 or 16 bit register or memory location. The source can be either an immediate value or a register of the same size as the destination.
When testing based on a memory address using a register, the register can hold a larger integer which defines a doubleword in memory to test and a bit within the doubleword. So you can access an array of bits.
Some examples of using btc:
btc rax, 15 ; test and flip bit 15 of rax btc eax, 10 ; test and flip bit 10 of eax btc dx, cx ; cx contains bit number of dx to test & flip btc [x], rdx ; rdx contains bit number of x to test & flip