The Ninja records from the HDMI output of the camera. If you set your internal recording to 25P the camera outputs this as 50i, however it is recorded as a 25PsF in a 50i wrapping. In fact presently all Cameras output via HDMI at either 50i or 60i. There is not a camera that outputs native 25P or 30P out of the HDMI port.
PsF is utilized in some DV, HDV and AVCHD camcorders for 25-frame/s and 30-frame/s progressive-scan recording, and can be called:
Progressive recording (Sony)
Progressive scan mode (Sony)
Frame mode (Panasonic and Canon)
Digital Cinema mode (Panasonic)
Cinema mode (Canon)
So when your camera converts from 25/30P to the interlaced system (50/60i) it splits that frame into two, producing 2 fields each field has the same image (the image does not change between field 1 and field 2). Each field contains 540 lines, the first field scans the odd numbered lines and the second field scans the even numbered lines.
When this is put on the editing timeline (e.g. FCP), it is seen as interlaced footage but of the same frame (in a 2:2 pulldown format to be technical) so basically the editor sees as interlaced frame however pulls it together as a progressive frame.
Please feel free to check this with your camera/camcorder manufacturer.