What is the red switch for on Kenwood Chef A901?

If the attachments that you are talking about are the dough hook and the whip, then the red lever is not used to put them in or take them out. The red lever releases the plastic housing on the end of the mixer, which then exposes the mounting hole for the major attachments such as the grinder, sausage maker, etc. You push the red plastic lever, then gently pull off the plastic end of the mixer. It should come off with absolutely no force. Then insert your food grinder or other attachment with the red lever held back. Once the cog on the grinder is aligned with the fitting inside the hole, release the red lever so that the recessed hole on the grinder will become engaged. The Kitchenaid classic stand mixer and the Viking stand mixer have essentially the same set up.