It's an IR battle sensor, unsure of what make (Tamiya TBU etc) as I'm not well-conversed in that dept. They generally have 4 "hit" LEDs, one facing front/back/left/right to detect incoming IR "shots" from other tanks. They get hit, tank slows down, get hit a few more times and they jitter about and blow up (well, stall.... wait 15 seconds.... tank restarts)