Sukuinage is a beltless arm throw ("sukui" is here "wrap arm under opponent's armpit", and "nage" is "throw"). The wrestlers will be facing each other when the winner, from an inside grip (his arm goes under the loser's arm), reaches his arm around the opponent's back while twisting away. The loser will be pulled forward and down.
Sukuinage is used as frequently as shitatenage (underarm throw) - and is very easily confused with it. Both throws are initiated from an underarm grip, but in shitatenage the winner will hold on to the loser's mawashi (belt) - in sukuinage that is not allowed.
The throw (as is the case with most throws) is favoured by the technically inclined sumotori (sumo wrestlers) such as Yokozuna Asashoryu and Sekiwake Kotooshu. The legendary Yokozuna Chiyonofuji was partial to sukuinage, and actually won his 1000th bout using this kimarite.
Of 571 bouts in the Makuuchi division (Haru and Natsu Basho, 2005), 16 (2,8%) were won by sukuinage.
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My sources are www.scgroup.com/sumo and www.sumo.or.jp/eng/index.html