In a short term memory task participants are asked to remember a stimulus, e.g. "XKF" and then, before recall they are often asked to count backwards. Why?

In a short term memory task participants are asked to remember a stimulus, e.g. "XKF" and then, before recall they are often asked to count backwards. Why?

This is known as articulatory suppression, which is a technique for disrupting verbal rehearsal, thereby eliminating the "recency effect".

After being asked to remember a verbal stimulus people are often asked to count backwards (usually it's made more complicated by asking to count out loud and by 3s) during short memory tasks to eliminate the possibility that the participants will use maintenance rehearsal (repeating the stimuli). Repeating the stimuli requires the use of the phonological loop which is a part of working memory and not short term memory which is being tested in this case.

Since the study is conducted to study short term memory, the experimenters want to eliminate the possibility that the participants could be simply rehearsing the information presented so they ask them to count backwards (out loud and by 3s) so that rehearsal becomes unlikely due to articulatory suppression. This works because both rehearsal and counting out loud depend on the phonological loop.

It is asked by the designers of experiment to prevent the recency effect, so repeating the obtained information in the mind. This works because both rehearsal and counting out loud depend on the phonological loop, which is a part of working memory. This way only STM can be measured.