Tsukiotoshi is a fairly commonly used technique. It is a thrust down technique, where the winner brings his opponent down, front first, with a thrusting motion ("tsuki" is to "push or hit opponent's body, usually with stretched arms", and "otoshi" is "make opponent fall onto his front or side"). The winner will push down on the loser's shoulder or upper body, while turning or stepping out of the way, making the loser fall forward. It is quite often seen as a last ditch move at the edge of the ring (the shobudawara).
This technique is very hard to distinguish from hikiotoshi and hatakikomi as the differences are minute. To tell the difference between a pull down, a slap down, and a thrust down is something that requires long practice. Actually a hikiotoshi may be changed in "mid air" to a tsukiotoshi. Even if it was a pull that caused the loser to come off-balance, it will still be ruled as tsukiotoshi if the winner helps the loser on his way with eg. a hand to the shoulder.
Tsukiotoshi does not seem to be the preferred kimarite of any one wrestler. It can be seen as a defensive move against a charging opponent, or, as mentioned: a last effort to avoid being forced out of the ring.
Of 571 bouts in the Makuuchi division (Haru and Natsu Basho, 2005), 26 (4,5%) were won by tsukiotoshi.
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My sources are www.scgroup.com/sumo and www.sumo.or.jp/eng/index.html