Its a technique called thrust bump and no longer in our FCTM. The logic is as follows:
The thrust levers don't move when A/T is active in the CLB detent. The actual thrust being commanded can be anywhere from idle to CLB.
The A/T disarms when moved to MCT, but rearms if moved back to CLB.
So if the pilot anticipates that the a/c is going to run low on energy (sinking feeling), he/she can preempt the A/T by momentarily moving the levers from the CLB detent to the MCT detent and immediately back again. You get a nice surge of thrust as the A/T disarms and starts to run up to MCT, then back towards idle (or stays near climb if your hunch is correct and the speed starts trending low) as A/T rearms when you place the lever back into CLB.
There are 2 big issues with the techinque. Firstly it is somewhat destabilizing if you do not have a good feel as to how much energy the bump will add to the approach. Second issue is the A/T do not rearm below 100ft, so if you do this below 100 you should not move the levers back to climb, but straight to where you need the thrust lever position needs to be as in a manual thrust approach. Quite a few have embarrassed themselves forgetting this change at 100ft and end up blasting into the flare at +30kts.
Personally I just use maunal thrust. More simple. More fun to fly.