"Sashikomi" is when you deal into another player on purpose (or try to do so) in order to keep your position. This becomes a viable strategy when you are on top and way ahead in points; losing a few points to a cheap hand is a small price to pay for skipping to the next round and denying other players their chance to catch up.
Screenshot 1: I am close to putting together either a tanyao (all simples) or chiitoi (seven pairs) and I decide to get rid of the 5p before it gets any more dangerous. It's actually this player's winner, but the hand is only 2000 points. This is not sashikomi, but a happy accident. I felt stupid when I dealt the 5p, but relieved when I saw the hand value.
Screenshot 2: With two riichis on the board and nothing that's safe against both players, I am specifically attempting to deal into the player on the right by discarding tiles that are safe against the dealer (they're in their discard pool) but not safe against the player on the right. It doesn't happen and the round goes to a draw, but this is a sashikomi attempt.
Screenshot 3: I deal 8s into player on the right for 6200 points. This is enough points that the player on the right is able to take second place, but not enough that my position is challenged. It's a mutually beneficial transaction. The game ends with me in first place. This wasn't intentional at all, but when I won it I laughed and said to myself "ah, there it is! the sashikomi worked!" This was not sashikomi.
Anyway, sashikomi won me tonight's match at the Galaxy table.