no need to leave the house.
A powerflush uses a machine which holds a vat of water, and a powerful pump. Hoses from this are attached to the pipes of your central heating system by disconnecting your circulating pump, and connecting the hoses to the pump unions.
A cleaning chemical is added to the water in the vat and it is then pumped backwards and forwards through your pipes and radiators. A filter or (more usually) powerful magnets catch the sediment as it passes through the vat. Meanwhile, the operator or his assistant goes round turning all the radiators off and on so that the water passes forcefully through each.
The operator will need access to the pump (usually beside the boiler or the hot water cylinder), the valves of all of your radiators, and a nearby drain or WC that the dirty water can be tipped down; and a tap (such as a garden tap) that he can put a hosepipe on.
He might splash some dirty water around the machine, but should bring floor coverings to protect it. Unless he has dirty boots he need not make the whole house dirty. He will need access to bleed all the radiators. If you have a feed and expansion ("header") tank in the loft he will need access to it. If the radiators are badly clogged, he might have to take them off the wall and hose them through in the garden, while vibrating them. If he has to go in the loft he will probably bring some of your dust and dirt down with him.
When cleaning is finished, he should add a corrosion-inhibiting chemical to the water on final fill to prevent future corrosion. Make a note of the brand and type used as you will need the same if you ever need to top up the system. He ought to stick or hang a label on the boiler or pump saying what chemical was used, and the date. Ask if you can keep the empty bottle or cartidge of chemical and put it somewhere as a reminder.