This sutra is to solve simple equations which has big numbers in it. See the example,
5x + 8y = 3
10x + 12y = 6
Here the x coefficients and the constants are in ratio, So we can consider the other one which is ‘y’ as zero.
The explanation mentioned above is a simple one. This sutra is also used for solving complex problems. It also has a sub- formula (sutra) ‘by addition and by subtraction’