what does checkpoint do in simple mode?

Since you just changed the recovery model to simple I would say that you won't loose nothing. But you can check that by the time that it's need to set the recovery model to full again.

And don't forget to make a full backup immediately after went back to full recovery model.

the above steps was recommended to 'regenerate the log file' which was corrupted.. hence I was trying to understand how checkpoint can do that?

if I just do
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database is in full mode.
  change it to simple
  change it back to full.
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it won't regenerate the log file, will it?