Generally you will want to inform participants of how they are doing. This is where the feedback item comes in.
The feedback item provides essentially the same functionality as the sketchpad item. So why not use a sketchpad item? The reason is that a sketchpad item is prepared well in advance, which is why sketchpads can be presented very rapidly. For this reason, if you present feedback using a sketchpad item, the feedback will generally not be up to date. In contrast, feedback items are prepared only when they are presented, which is why they are up to date and suitable for presenting feedback.
The following variables are especially relevant for presenting feedback:
response_time contains the response_time of the last response
response contains the last response
correct reflects whether the last response was correct if the variable correct_response was specified
avg_rt / average_response_time reflects the average response time since the last feedback item
acc / accuracy reflects the percentage of correct responses since the last feedback item
So a typical feedback item would contains something like the following string: 'Your average response time in this block was [avg_rt] ms, and your accuracy was [acc]'.