It sounds like a variation of subtracting that I learned in high school (1968). My instructor called it European subtraction.
\begin{array}{ccc}
& 3 & 4 & 2 \\
- & 1 & 7 & 3 \\
\hline
\end{array}
You start by saying $9$ plus $3$ is $12$.
You write the $9$ as shown and "carry" the $1$, of the $12$, as a subscript of the $7$ as is shown below.
\begin{array}{ccc}
& 3 & 4 & 2 \\
- & 1 & 7_{\color{red} 1} & 3 \\
\hline
& & & \color{red} 9 \\
\end{array}
Then $7$ and $1$ is $8$ plus $6$ is $14$.
\begin{array}{ccc}
& 3 & 4 & 2 \\
- & 1_{\color{red}1} & 7_1 & 3 \\
\hline
& & \color{red}6 & 9 \\
\end{array}
Finally, $1$ and $1$ is $2$ plus $1$ is $3$.
\begin{array}{ccc}
& 3 & 4 & 2 \\
- & 1_1 & 7_1 & 3 \\
\hline
& \color{red}1 & 6 & 9 \\
\end{array}
The big advantage of this method is that you can do many subtractions at once.
\begin{array}{ccc}
& 5 & 3 & 4 & 2 \\
- & 1 & 9 & 2 & 3 \\
- & 2 & 9 & 4 & 5 \\
\hline
\end{array}
$5$ and $3$ is $8$, plus $4$ is $12$.
$5$ and $2$ is $7$, plus $7$ is $14$.
$10$ and $9$ is $19$, plus $4$ is $23$.
$4$ and $1$ is $5$, plus $0$ is $5$.
\begin{array}{llll}
& 5 & 3 & 4 & 2 \\
- & 1 & 9 & 2 & 3 \\
- & 2_2 & 9_1 & 4_1 & 5 \\
\hline
& & 4 & 7 & 4 \\
\end{array}
I think this is a wonderful way to subtract, but, I would not teach it to grade school students.