Using the Humax TV Portal, an iPlayer programme is buffered (in its entirety) in the process of viewing. You can pause iPlayer playback and the downloading will continue, or switch the TV (not the Humax!) to watch from another source, but you must allow the whole programme to download before stopping or exiting the playback otherwise the download in the buffer will be incomplete. Neither can you disturb playback in any way (other than pause/play) - using skip forward and skip back will corrupt the download in the buffer (regardless of it working properly in the player).
You can tell when it has completed by the status in the "save last streamed content" button (WebIF >> Browse Media Files) - the figure shows how much data is in the buffer, and when it stops growing the download is complete. Clicking the button then provides a dialogue box where you can give the download a name (it will be a .MP4), and the file will be saved in the folder you currently have open in the media browser.
The download proceeds at the speed of your broadband connection, so this is handy for a slow connection which would otherwise play in fits and starts - including HiDef downloads. Ditto when not wanting to save the download - pause for a while, then resume playback when an adequate buffer has built up.
The same applies to the TV portal YouTube app.
Which reminds me: I have the iPlayer exclusive "The Rat Pack" sitting in my buffer, waiting to save off.