to have a realized yield equal to its YTM, all cash flows will be reinvested at the YTM and assumes that the bond must be held until maturity. Also in schweser notes book5 P128, Q8, selection B., why the statement is wrong? - I understand the reinvestment yield equal to YTM, but hard to accept bond held until maturity. For example, a 3 years option-free bond coupon rate=YTM=8%, issued at par, semiannual pay 1year later, YTM is still 8%, investor sold it at $1000, and reinvested the coupon at 8%. In this case, the realized yield is 8% equal to YTM. Just in the case of interest rate volatiles, bond must be held until maturity for a same realized yield as YTM
it might help if you think of YTM in terms of IRR
This is one of those sufficient not necessary things.