Kotenage is an armlock throw. It is often seen used as a counter-attack by a sumotori (wrestler) who has this one chance before being forced out of the dohyo. He will wrap his arm around the opponent's inside gripping arm, lock it up on the elbow and then turn away, forcing the opponent down ("kote" is "wrap arm over opponents upperarm", "nage" is the throw). The loser will either fall - or have his arm broken.
Kotenage and Tottari (arm bar throw), are techniques where the attacker only holds on to one of the loser's arms. In kotenage the loser's arm will be behind the winner's back - and in tottari, it will be across his chest.
As most other throws either require a grip around the body or in the mawashi (belt), or involve using the legs, kotenage (and tottari) are relatively easy to spot. A -dashi-nage (a pulling throw from either an uwate- or a shitategrip (overarm-grip or underarm-grip)) can be mistaken for kotenage (or tottari); the trick here is to see what the winner is holding on to: body or arm.
Though kotenage is a good last-ditch effort, it can be executed at any given time during a bout. When done well, the kote-grip is very efficient, as it locks up the opponent's elbow. Short of dislocating the shoulder, there's no getting out of it.
Of 571 bouts in the Makuuchi division (Haru and Natsu Basho, 2005), 7 (1,2%) were won by kotenage.
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My sources are www.scgroup.com/sumo and www.sumo.or.jp/eng/index.html