In the blutetooth spec. This trigger signal is used to determine the receive timing.
Correlation is a very useful method to match a known signal with a possible future one.
The method is to use the known signal as a 'match pattern' to match the uncertain signal, if the correlation value exceeds the trigger level, we can say we have found that signal. When the signal is coming, the correlation value's trend in the sliding correlator is from low to high. When the signal is passing by, the correlation value's trend in the sliding correlator is from high to low. The Trigger level uses the Access Code located in the first part of the baseband packet. When the receiver is triggered it means that the receiver has found the packet header.