SMA theory

When 2 SMA crosses each other or when 2 EMA crosses each other, there is buying and selling position depending on the higher SMA or the lower SMA crossing from above or below.

what is the theory behind this ?

many thanks for advice.

He is explaining about the buying and selling positions occurring in the market. When the 2MA or the 2 EMA crosses in the market there you can see a buying and selling position through which you can decide your trades

say example SMA 10 crosses SMA 30.

one higher ma - one lower ma.

why when they cut each other , then buying or selling position will come in ?

When a 10 SMA/EMA crosses through a 30 SMA/EMA it simply means that the average of the last 10 bars is greater/lesser than the average of the last 30 bars. Some see this recent activity as a signal to go long/short.