it moniters the battery voltage and interupts the esc so the batterys do not get over discharged, which will ruin the batterys, i used one from novak, and it was very sensitive to back to back hard runs , which lowers the pack voltage and shut the esc down, i dont remember which one i used , i think it was the 2s one, if interested in the novaks email them directly, they will also let you know where to solder it on the esc. , but there are many available from other places