Y- cable for the shift sensor also called a gear sensor. Some Bafang motors do not come with a shift sensor cable from the back of the motor. If this is he case with your Bafang BBS02 750 watt or your Bafang BBSHD 1000 watt electric bike motor kit you will need the Y- cable for the shift sensor. This is also called a Y splitter cable. What this cable does is it allows you to plug in your shift sensor into the front of your bike and to put the shift sensor near the shifter itself. The normal cable for the Bafang is two yellow Higo connectors for the eBrakes. The Y- cable plugs into one of these connectors. Because it is a Y cable you to have one for the eBrake and another for the shift sensor. Pretty simple fix.
There are still a lot of the older Bafang BBS02 750 watt and the Bafang BBSHD 1000 watt kits available. When purchasing one of the electric bike motor kits ask if it has the gear, or shift sensor, lead at the back of the motor. If it does not have this lead then it is an older style motor. That does not mean it is not a good motor. If you purchase a motor without the lead coming from the motor you will need this Y cable.