“the plain.” The Jezreel Valley at the foot of Mount Tabor is actually a wide plain, and the Hebrew word can mean “valley” or “plain.”
“behind him.” The Hebrew is more literally, “at his feet,” using the same idiom as in Judges 4:10.
“Among the divisions.” The Hebrew word translated “divisions” can refer to the divisions of a tribe, thus “clans,” and it can also relate to streams of water, such as “by the watercourses.” The Transjordan tribal area of Reuben was divided by deep valleys that had water running through them, and those natural valleys may have divided the people of Reuben into different clans or groups. It seems that the word was purposely used to portray the different groups in Reuben.
“deliberations of heart.” There is some manuscript discrepancy, and some manuscripts read “searchings,” but the Masoretic Text is more difficult and likely to be original. Furthermore, Judges 5:16 has “searchings,” and it is common in Hebrew poetry that the same fact is stated in two different ways. In this case, “deliberations” would be in verse 15 and “searchings” is in verse 16. The people of Reuben thought about the situation and the plight of their fellow Israelites, but did nothing about it.