Swinsian uses a queue to manage which tracks will be played next; this list of upcoming tracks can be seen by clicking 'Playback Queue' in the left hand side of the main window. The track that is currently playing will be shown at the top, followed by the tracks that will be played next. These tracks can be rearranged by dragging or sorted by clicking the column headers.
To insert tracks into the queue, select them and select 'Play Next' from the right-click menu or 'Track' menu. The 'Add to Playback Queue' command will add tracks to the end of the queue. 'Play Now' will play the track(s) immediately, then continue to play the tracks already in the queue.
When you double click a track in a playlist or in the 'Library' view it begins playing and any tracks after it are added to the queue, replacing the current queue contents. If you then rearrange or sort the playlist you were viewing this will not alter the order of the tracks that have already been queued; to change the order of upcoming tracks select 'Currently Playing' and then rearrange the tracks.
The tracks in the Currently Playing queue can be shuffled by selecting 'Reshuffle Queued Tracks' from the 'Control' menu, or by pressing the shuffle button in the main window toolbar. If the shuffle button has been toggled, double clicking a track will begin playing that track and add the other tracks in that playlist to the queue after shuffling them.