Why do manuals always state that you should turn on the heated rear window, etc. on the car with the flat battery before disconnecting the jump leads?

I know it's to avoid voltage peaks but I'm hoping for a more detailed explanation.

 

Sad i know, but I was thinking the EXACT same thing in the shower this morning

I also wanna know why they say you are supposed to put the neg lead on an earth point in the engine bay, and not straight on the battery?

 

I think the latter one is because the battery can emit explosive gasses and you don't want to create a spark near it.

 

i don't know, but do know not to do it on new mondeos/s-max/galaxy as nine times out of ten the abs module takes the voltage spike and kills it!

 

It is because some cars electronics are not very well protected, you can buy protector devices that slowly(still quickly) let current flow to flat one to stop this effect.
There is NO problem with connecting a jump lead to a flat battery as SG will be so low as not to emit hydrogen gas anyway, it only does this upon being charged, but the reason of explosion would seem the most logical one.
tabetha