You are correct that you're totally wrong. We don't vary the angles over time, we just select a large number of angles. Basic idea, if you shot from just one angle, all the tissue on the entrance path would be getting more dose than the tumor. If you shot through 10 different angles for a single treatment, than each entrance path through normal tissue would be getting only 10% of the dose, but where they all overlap at the tumor, it's getting 100% of the dose. This treatment with the 10 angles (usually actually only 5, 7, or 9 for static IMRT) would be given every day for 5-8 weeks in conventional radiotherapy using those same angles for each daily treatment.

The evolution of this is having the gantry rotate constantly through the treatment, varying the dose rate and shape of the beam during the rotation to avoid dose to especially sensitive tissues, while making sure you completely cover the target. So instead of 10 (or 5 or 7) discrete angles you have the maximal spread of the entrance dose. It's called VMAT, volumetric modulated arc therapy.