Originally Posted by Russo2
What exactly is it on the valve that fails?
The valve itself doesn't fail; the valve seat insert fails. The valve seat insert separates from the cylinder head within the combustion chamber and gets trapped between the moving piston and top of the combustion chamber with disasterous results.
"Soft" aluminum heads have hardened circular valve seat inserts - to withstand the "hammering" of the valve rapidly opening and closing - embedded into the head. They are usually held in place by a simple interference fit; the seat fits tightly into a counterbore machined in the head.
So any valve seat repairs require head removal and complete head dismantling (cam(s) off, valves and springs out) with some machining involved.
edit: amc49 beat me to it.