I assume that you want to reduce the size of the transaction log file? - Is that your goal? If this is the case then backup the log file. Use dbcc loginfo to see the active parts of the log file, as it gets reused by your app. Using dbcc shrinkfile (or ent manager) you can then eventually reduce the size of the trans log file.
All that checkpointing does is flush all the committed tranactions back to the datafile(s). It does not impact the size of the transaction log file.