I've heard a martial arts Sensei use a phrase that sounds to my English ears like "yow may", I think in context (at the end of a teaching, when it was time to practice he would clap his hands and then say this) meaning something like "let's go" or "to the front". What's the proper/standard spelling of this phrase when you transliterate it to Latin characters?
What you are referring to does indeed sound like (止め) or Yame!! A lot times (止め) or "stop", is used in some sort of practice like baseball (少年野球) ,budou (武道) and other sports.
In a competition, Judo for example this is what is said.
Before initial match. 『まて』、『そのまま』 『Wait』、『stay』
Re-start of match and during the match. 『はじめ』, 『よし』 『Start』, 『resume』
At the end of the match 『それまで』 『Stop』Not the literal translation, that would be 『Until then or the end』
Although (止め) is used quite a lot during pratice it is not said in a tournament.
Not sure if that helps you any?
As naruto said, it could be "yame" (止め), meaning "Stop!", "Avast!". I disagree that it would have to be after the practice to make sense. Having no other suggestions I will go with this.