1) A buzzer sounds every 10 seconds. The player whose turn it is to move must make his/her move while the buzzer sounds (two seconds); move on the buzzer, not before, not after.
2) The FIDE Laws of Chess Articles 1-5 apply.
3) Completed illegal moves lose.
4) A move which is made after the buzzer has finished sounding (a “late move”) is treated as an illegal move and allows the opponent to claim a win.
NOTES
a) An attempt to replace an (incomplete) illegal move with a legal move (cf touched piece rule) will probably amount to a late move (see Rule 4 above).
b) A king left in check should be claimed as an illegal move. [Earlier versions of Lightning rules used to say ‘capture the king’ but capturing the king is an illegal move, so would lose! Claim the original king left in check as an illegal move.]