Enter BIOS, change Long Term Power and Short Term Power to lower values like 35W and 45W respectively and you'll have a CPU that will dynamically clock to fit into 35W TDP. If you set Short Term Power at 45W like in my example, then it will use 45W for a limited time duration before dropping down to 35W: this is useful for burst loads of 15-30 seconds. The time interval in which Short Term Power is allowed can also be configured.
TDP can be configured as low as 20-25W. The CPU will use the highest frequencies it can under any load, the highest allowed by your multipliers ofc. If you take the 8700 and configure it for 35W TDP, it will boost to it's max frequency for 1-2 threads, but higher thread counts may end up running at lower frequencies than stock, depending on power usage. To make the most out of a low TDP setting you may want to make sure C states are enableb in BIOS, not just set to Auto.
Las but not least, you can also overclock while running a lower than stock TDP. In theory you can have a 5Ghz 8700K @ 65W TDP: it will boost to 5Ghz as long as power usage is bellow the limit, then drop clocks to fit the thermal envelope.
This type of power management has been around on Intel CPUs ever since Sandy Bridge. It got refined and gradually exposed to the user with every new generation. It offers excellent flexibility and is the reason why I always try to make sure the boards I purchase expose the features via BIOS. (probably all Z boards do, but I would check on cheaper models)