The important thing to remember is that the "afterload" on the heart is the diastolic blood pressure in the aorta. In other words, the pressure in the aorta that the heart must overcome to open the aortic valve and create a pressure gradient to force blood into the aorta and generate flow. The isovolumetric part of the contraction is the generation of pressure up to the afterload 'pressure'. Once the pressure in the heart is higher than the pressure in the aorta a pressure gradient is generated and the aortic valves open and blood flows out of the heart into the aorta. During this movement of blood the heart is contracting again and changing volume (i.e. not isovolumetric anymore).
If the afterload is higher (i.e. higher blood pressure or hypertension) it increases the work on the heart because it has to generate an even higher pressure within the heart to open the aortic valve to generate the pressure gradient and flow.
If the afterload is lower (i.e. low blood pressure or hypotension) the work on the heart (with all other factors remaining constant) would decrease because the pressure to open the aortic valves and create flow is lower. The diagram below is available online at this link. This is a very common picture/diagram that can be found in any cardiovascular physiology textbook.
Note: In the picture below the isovolumetric contraction is only the time between the two vertical lines as indicated. During that isovolumetric contraction you can see that the volume (i.e. the red line) stays the SAME, but the pressure in the ventricle significantly increases up to the point of aortic pressure.