Originally posted by madhavb
Can I do it here?
You can, but it's considered poor sportsmanship to do so.
I'm just kidding.
It's a perfectly legitimate chess move.
Essentially, if you have a pawn on your 5th rank and the other player moves a pawn on an adjacent file from his second rank to his 4th rank (i.e., jumping over the square at which your pawn could have made the capture), you can make the pawn capture as though the opposing pawn were on his third rank. This can only be done at the first opportunity, however.
If you think of the logic behind it, the rule becomes simple. Double pawn moves from the 2nd to 4th rank were meant to speed up the game by giving pawns a head start, not to enable pawns to jump ahead to avoid capture by other pawns. So, if the pawn tries to jump with a double move past where it could have been captured by another pawn, you can capture it anyway.