Your lower body with its legs out is hard to rotate, so it rotates a little the other way, from 100 degrees to 90 degrees. Bring your legs together and land Both parts are facing east. At all times the sum of the angular momentum of the upper and of the lower body have cancelled, that is the sum is zero. The moment of inertia measures how hard it is to change the rotation of an object. It depends on the mass, m, and the distribution of the mass.
Different objects have different moments of inertia.
The result of one rotation followed by the other is that the body ends up rotating through 90 degrees while at all times its angular momentum is zero.