Suppose you have two tasks A and B both of 1 day duration (they start the same day). I set start-to-start dependency on them, which means that task B can't start until task A starts.
Then I set the lag to -1. Now task B starts before task A.
Can someone please explain it to me? I am a little bit confused.
You started out saying that B can't start until A starts. What you've actually said is that B can't start until (at the earliest) 0 days after A starts.
Ok, now change the number to -1. There are many ways to read it...
B can't start until (at the earliest) -1 days after A starts.
B can't start until (at the earliest) 1 day before A starts.
A can't start any later than 1 day after B starts.
A must start within 1 day of B starting.
Rob F explained it quite nicely, but let me put this into an example:
Imagine you're doing alfresco decoration.
You are decorating a wall, which needs to be plastered (B) and painted (A). However, you can only paint in semi-dried (half-wet) plaster. Hence, you must start plastering before decorating, but not too long before it, because then it will have dried too much. It's delicate timing.