Bypasses, radiators without TRVs providing a bypass, automatic bypass valves...
all of these do the same thing and for most boilers are ESSENTIAL.
ESPECIALLY with a 'low water content' boiler (aluminium heat exchanger, physically small), the boiler builds up quite a lot of 'latent heat' when it's running. This heat acts like a flywheel - if everything keeps going (pump, burner, flow of water round the plumbing and through the rads, ... - it all stays in-balance and the boiler is happy.
But if the pump stops or the flow through the rads is stopped or slows down then the main thermostat in the boiler, and / or the secondary overheat stat, will operate and switch off the burner. OK - so there's no more heat going into the boiler but there's LOTS already in there. If there's no water flow to carry it away, the boiler will overheat and boil - producing banging noises, water and steam out of the emergency discharge pipe from the Pressure Relief Valve (PRV), pressure loss and general Bad Things for the boiler.
Usually, all that's needed to prevent this is a loop through the plumbing to allow the pump to carry on taking hot water out of the boiler through the Flow pipe and cool it down a bit before putting it back in through the Return while the boiler cools down. This, by the way, is also why most boilers have 'pump overrun', so that the pump carries on going for a couple of minutes after the burner has switched off. In many cases, the pipe run between boiler and HW cylinder is actually a big enough 'heat sink', which is why you'll often find an 'open bypass in the plumbing near the cylinder. (Note that if there's a valve on the bypass, it should be adjusted to allow enough flow to dissipate the heat.) Another bypass design uses a radiator or, more often, a towel rail as a bypass - which is why the control valves on whichever MUST NOT be turned off. Other systems include an 'automatic bypass valve' which is a special spring-loaded valve linking flow and return some distance from the boiler. This is adjusted so that if all the other paths between flow and return are closed (by TRVs, zone valves, etc.) it opens to provide the bypass route to cool the boiler.
Combis are a bit different, mostly I guess because they work much harder in HW mode compared with CH. Some have 'internal bypasses', others actually use the fan rather than the pump to dissipate heat. Some include a jumper option to select either pump or fan (Ariston Microgenus owners take note: this model is a bit susceptible to overheating in HW mode when the heating is completely off and seems to work best with fan overrun selected.)
End of lesson on bypasses.