In binary subtraction i have seen some tutorial and some sites that during 0-1 process borrow value from next 1s.
Let's say one of tutorial is this
http://courses.cs.vt.edu/csonline/NumberSystems/Lessons/Subtraction/
It's first rule
To compute the first column, we need to borrow a 1 from the next column. Recall that two 1s generated a carry in addition. If we reverse this process, we can borrow a 1 from the second column and mark two 1s in the first column.
Why carry two 1s generated a carry in addition ? Can anyone please explain ?
It looks like you are doing $10_2 - 1_2 = 1_2$ Remember what the positional notation means in base $2$: Each place is twice the previous one. Expressedin base $10$, this is $2-1=1$. When you do $$\begin {align}10_2&\\ \underline{-\quad1_2}&\\ 1_2&\end {align}$$ you recognize that the $1$ in the twos place in the top line represents two in the ones place (but you don't have a symbol for that in base $2$). It is exactly the same as $$\begin {align}10_{10}&\\ \underline{-\quad6_{10}}&\\ 4_{10}&\end {align}$$