HA A-A distribution on load

hello fellas,

Anyone could shed me more info regarding distribution of load from HA A-A. I am aware that in HA A-A the Master will handle all the traffic and forwarded to Slave to load balance the traffic flow.

From what I've read, still cant get the concrete answer in what in instance that the Master will tell himself that I need to forward this traffic to Slave? Is there certain threshold, performance basis or criteria?

any thoughts are welcome. thanks in advance