Bit Number to Byte Value conversion (AVR Docs)
The bits within a byte are numbered 0 through 7 from right to left. The
rightmost (least significant) is bit 0, next is bit 1... and the
leftmost (most significant) is bit 7.

The expression _BV(3) builds a byte where bit 3 is set (is 1) and all
other bits are clear (are 0). In binary it looks like this:

position:   7  6  5  4  3  2  1  0
bit value:  0  0  0  0  1  0  0  0


The macro expands to 1&lt;&lt;3, which takes the integer 1 (0b00000001 in
binary) and shifts all its bits by three positions to the left. The
result can be written 0b00001000 in binary, 0x08 in hexadecimal, and 8
in decimal.