I believe one holds ones tongue to the roof of ones mouth to activate one of the major energy centre's of the body. It acts as a gate (on/off)to this large centre. A lot of chi gong and other martial art practices recommend this as well.
The idea behind the altered exercises is to increase the effect they have as well as allow everyone to perform them, no matter what there age or health level is. Doing 9 of each excersie also allows them to be peformed much more quickly than having to do 21.