Fast multiplication of two digits by 9!

I came across this trick in this video

It's a neat way to multiply by nine when the ten's digit is smaller than unit's digit for example like 23,,27, 38 and so on but not for 32,72,or 83. So how does it work.

So lets multiply $47 \times 9$

1. Write the first digit --> 4
2. how many digits in between the two digits --> 2  {5,6}
3. number of digits to make last digit reach 10--> 3 {8,9,10}

Therefore $l 47 \times 9=423$

Another quick example $ 29 \times 9 = 261$