CHDK is custom firmware that is loaded from the SD card so you dont have to replace the current firmware.
It does enable raw but it isnt really a proper raw format as it has no header and as such cant be read by any raw processor. They recomend using DCraw to read it and one such RAW processor that uses this is Raw Therapee.
There are also scripts that can be run using CHDK for motion detection that starts a shot. and also longer shutter speeds
it looks like a great addon for canon camera's not all models are covered but theres lots of info.
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