The Toppy has two tuners so you can record two channels at once. Most PVRs will then limit your viewing to one or either of those channels, but the Toppy has a very clever chipset — you get a much wider choice and sometimes all channels are available (this is often referred to as "Record Two, Watch a Third"). Technically, this choice is dictated by which multiplex (or mux) the two recording channels are from. If they are from the same mux then the Toppy can use just one tuner, leaving the second tuner free for watching any channel. If they are from different muxes then the choice of channels that can be watched will be limited to any channel that is also on one of those two muxes.
For example, (in the current Freeview setup) if you are recording BBC1 and ITV1 then you cannot watch Five. But you could watch, say, BBC2 or Ch4 (as well as BBC1 and ITV1, of course). However, if you are recording ITV1 and Ch4 then you will be able to watch any other channel.
Note that there are bugs/features which sometimes make it seem like the Toppy is more limited than this, causing "CYR" prompts at unnecessary times.
If recording two programs and watching a third the 5800 can sometimes become unstable and require switching off, usually after using the remote control to do something. This may be more likely if several TAPs are running. If this happens, in future try not to change the watched channel when recording two others to avoid spoiling the recordings.