Thanks for the reply. Yes I meant "affect". And I realise I have indeed got a bit confused about the wings being level!
I guess if you keep the site picture the same, the increase in descent actually means the airspeed is staying the same, but you're trading distance horizontal through the air, for elevation?
OK I've just realised that you lower the nose, to maintain the same airspeed in the slip, and that is why your rate of descent increases. Sorry I think I understand it a bit better now, keeping the same site picture relevant to the horizon would slow the aircraft down, which you wouldn't want if you're already at the correct approach speed, so you would lower the nose to keep the desired speed?