It's thought that China got F-35 plans via hacking into the contractors. That would be enough to understand the board. Actually having the board means they can

It's thought that China got F-35 plans via hacking into the contractors. That would be enough to understand the board. Actually having the board means they can embed a chip inside it. Combine those, and they can MITM all sorts of data. Depending on the purpose of the board: They can effectively grant stealth to their non-stealth aircraft. They can cause the F-35 to emit a response to a coded message. Without much knowledge of the board, there is still hope for causing mayhem. The modifications could check for kill codes on many traces. China could deliver these in a multi-spectral way, not knowing what data actually flows through the board. So the kill codes show up on all the RF frequencies that the F-35 is thought to receive, and on anything optical, and so on. The kill codes could then disable the board, supply high-voltage pulses to other boards, or perhaps even make the board detonate.