Please answer a question about the following article about Southeast Asia:

The climate in Southeast Asia is mainly tropical–hot and humid all year round with plentiful rainfall. Northern Vietnam and the Myanmar Himalayas are the only regions in Southeast Asia that feature a subtropical climate, which has a cold winter with snow. The majority of Southeast Asia has a wet and dry season caused by seasonal shift in winds or monsoon. The tropical rain belt causes additional rainfall during the monsoon season. The rain forest is the second largest on earth (with the Amazon being the largest). An exception to this type of climate and vegetation is the mountain areas in the northern region, where high altitudes lead to milder temperatures and drier landscape. Other parts fall out of this climate because they are desert like.

What causes additional rainfall during monsoon?
The tropical rain belt