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This is a periodic checkpoint. This can be specified with an interval measured either in a number of increments (i.e., passes over the image), or by the elapsed wall clock time. For example, you could ask the renderer to write checkpoints with an interval of 100i, meaning every 100 increments and it will update the images on disk with the state of the render on the 100th, 200th, 300th increment, and so forth.

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Both methods for generating checkpoints can be used together. For example, it is possible to request a checkpoint be written every 100 increments until 15 minutes has elapsed. At that point, any periodic checkpoints that were written will simply be replaced with the final checkpoint when exiting.

Note that checkpointing is designed for batch rendering to images on disk. Renders to a live framebuffer such as "it" are already updated on-the-fly as the render proceeds. Of the built-in display drivers, currently, only the TIFF and OpenEXR display drivers support checkpointing. You may notice that the files produced include a ~w labeled channel. This is data stored to resume a render from this file.