Armed with the new machine,a search party went into the cave

Armed with the new machine,a search party went into the cave hoping to find buried treasure
为什么把 armed with the new machine 放在句首,而不放在a search party后面做过去分词做后置定语,这里运用到什么语法吗?请英语高手解析下 小弟感激不尽!