The standard method is to use a single cable into the sub LFE input. Alternatively, you can use a Y-splitter and jack into the L+R input - this will make the output increase about +3 dB as mentioned earlier (you can get the same effect by adjusting the sub +3 dB in your AVR). If you already own a Y-splitter then use it. Otherwise, it's not necessary to specially buy one.

Neither, the increase comes from the Y-splitter itself as a result of 2 signals being fed into the L+R input of the sub.

In summary, there are 3 ways to get an increase of +3 dB:

1. Turn up volume on the sub
2. Turn up sub adjustment in the AVR
3. Use a Y-splitter

Since you can do 1 and 2 easily, it is not essential to get 3. So then what is the use of a Y-splitter? Here's a scenario when it can help:

a) Some subs have auto on feature, when the sub detects the signal from the AVR it will auto turn on. If the signal from the AVR is low, sometimes it won't be detected and the auto on won't kick in. Using a Y-splitter increases the signal strength by +3 dB so easier to detect and the auto on will function. Of course the easiest is just increase the sub adjustment in AVR by +3 dB. Exactly the same effect. Maybe those people who have old receiver without sub adjustment may need the Y-splitter.