[Yaffs] how sheckpoint works？

i'm reading the yaffs2's sourcecode,and i'm puzzling about the check point
mechnism.
1.in struct yaffs_dev ,it's member checkpt_buffer,it seems storing
checkpoint data,and checkpt_byte_offs stores the offset of each check point
data.(i'm right?) in funciton write checkpoint to yaffs_dev and read
checkpoint from yaffs_dev,after write/read check point
data,checkpt_byte_offs always increace, this is why i'm puzzling,can
someone tell me why,thank very much.
2.as the checkpoint data is not write to flash immediately,if suddenly the
device lost power and the checkpoint has not been synchronized while many
successful opeartion has down.then reboot the device ,how does yaffs2 work?
use the checkpoint data to recover or scan flash to recover? and how
yaffs2 detectes?

thanks very much for helping me .

forgive my doll.