I asked BenR about them and he replied with this:
The SMT6 is a piggy back ECU. Basically you wire it into the the existing loom and it alters the signal from sensors to the ECU. The CU takes this data as it normally would and makes ignition/fueling changes as per normal. The difference is that you are giving t different signals than are occuring. This allows you to fine tune the management of fueling and ignition for optimum performance disregarding emission compliance mainly that the manufacturers need to retain.
Prices are rougly 400 fitted into a clio and then you need some rolling road time to set it up perfectly.
Its not like a pre-programmed chip which costs 350 odd bhp, these take no account of anything and just up fueling a few % in certain areas, a very crude solution. The SMT6 will allow you to pop back and get it re-mapped and updated to work with any other modifications that you might make later on.