The original code used mul in assembly to delay 2 clock cycles. As per the comments in the original code, the code needs to be compact and this 2 cycle delay can only use one word of instruction space. But the mul assembly instruction is not available on the attiny 85. The rjmp (relative jump) instruction can be used to do a jump to the next instruction, making it a nop. It requires two clock cycles, thus maintaining the correct delay. It is an instruction provided on the attiny85. So it makes a perfect replacement for mul.