As well as switching out the current task stack to put it into the pool
of tasks, you can create a new, one-level, task stack with the command
sidebrain-reminder, ready to resume later.
To make a reminder, type sidebrain-reminder. You will be
prompted in the minibuffer for the text of the reminder, and then for
a project group and project to put the reminder in.
The new task must be assigned to a project – see Working with projects for an introduction to sidebrain's project system.