What happens when a bike 'boggs'

I know extended riding at too low RPM's can be bad for the bike but what exactly happens to the engine/gear box?

Not sure, just seems to vibrate and shudder which would eventually lead to something coming apart. Maybe.

You're question is a bit vague. Are you talking about a bog as in when the bike doesn't get enough fuel for whatever reason and slows, or are you talking about lugging the bike at lower RPM's on purpose? In the first case it's typically more or less what I said, the bike not getting the proper amount of fuel for one reason or another. In the second depending on the engine design you could run issues with the engine not getting enough lubrication or any number of other potential issues that I'm not currently thinking of.

I wouldn't do it in the long run. it's fine if it's just for a few moments, but I wouldn't run 80 miles with the engine "bogging" (by bogging I usually understand as hyperboarder said when you suddenly open the throttle and the bike doesn't get enough fuel and struggles to gain revs. this usually only happens on the WR if the engine is cold)

so let's make sure we mean the same thing here. Running the engine at low rpm is fine as long as it doesn't start to run uneven. once it starts to stutter you should definitely down shift. The problem during the "bogging" is that for one, you have little lubrication as you are running at low rpm, but the load is still high. but more importantly is that you are unevenly loading your bearings. instead of a continuous load, you have a cyclical load which is not exactly good for the bearings.

It is caused by a high manifold pressure at low rpm which can and will turn into detonation.

OK. I did mean low RPM "lugging".

Thanks.

Lugging damage in the past was from not enough oil pressure and things moving too slow allowing metal on metal contact where there would otherwise be a fluid barrier. I think thats less of an issue on new tighter clearance motors but the vibration is still hard on things and at low enough rpms you can probably still get metal on metal, especially with the help of the vibration bashing things around. Easy to cure though, you just downshift.