Isamiashi is one of the five winning non-techniques. It is an inadvertent forward step out ("over-eager foot"), where the loser misjudges the distance to the edge of the ring and puts his foot - or just a toe - on the outside of the ring.
Before the 2001 winning technique list expansion, isamiashi was translated as just: "inadvertent step out". With the expansion of the list, isamiashi became "forward step-out", and fumidashi was introduced to cover "rear step-out".
Isamiashi does not get recorded very often. The "technique" is not one of the 82 kimarite, since it cannot be said to be a "winning technique", but rather a "losing technique".
Of 571 bouts in the Makuuchi division (Haru and Natsu Basho, 2005) none (0%) were won by isamiashi.
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My sources are www.scgroup.com/sumo and www.sumo.or.jp/eng/index.html