Our procedures to manually raise the speedbrake before initiating reverse thrust were introduced in 2008.
The reason given was due to a tech insertion on later model engines (improving gas flow). This required a new FAA test program that included new requirements for load testing.
The load analysis tested effect on engine components when during an RTO there is limited time for spool down before reverser sleeves translate. If using the auto speedbrake method, the sleeve movement may exceed the design load case. When using the manual speedbrake method, the design load was not exceeded (only an extra second of spool down is required).
Thus the change to SOP's, to require manually raising speedbrake first.
There is no change to field performance as the manual method is used during certification.