406 engine managment light question?

Can anyone answer this question for me?
if I brake as normal, you don't get the engine managment light come on, but
if I brake at the same time as the throttle being slightly on, the light
comes on.
I know this should not be done, but did notice it after I changed the front
pads and wanted to bed them in a bit quicker on slightly worn discs, I
don't think it's a problem but just wondered why it comes on?

 

same on mine. I think the engine realises there's something holding it back,
so it tells you.

Neil