Yes, the rule is simply to prevent people from doing this:
Ninja one is performing handseals and starts shooting a fireball at ninja two.
Ninja two does one handseal and makes spikes erupt under ninja one before he can finish his handseals and perform the fireball.
By doing this, ninja two cancels out ninja ones whole move, even though it was Ninja one that was going to move first.
This applies to first jutsu of every move, if the opponent doesn't perform a jutsu, but instead does some other action, it applies the same way, if the opponents first thing is to jump, you cannot prevent him from jumping by doing a jutsu before he jumps up.
So the rule says, that Ninja two's first jutsu cannot finish before ninja ones first jutsu, (first jutsu means the first action of every turn).
Been similar examples in the past, where people use a technique that stops the opponent from summoning animals, whatever.
Ninja one performs Summoning technique.
Ninja two slams the ground before Ninja one, cancelling his summon.
There Ninja two breaks the time-frame rule, being that he cancelled out the opponents move, and prevented him from doing his first technique, even though he was starting first.
I hope that is very clear.
If Ninja one throws a shuriken, and then does another jutsu, the shuriken counts as his first technique, so that means that ninja two can perform some spike jutsu that makes spikes appear under ninja one as he is about to perform the "another jutsu", right after he has thrown his shuriken.
You don't have to wait till the opponent is finished doing his move, but you need to wait till he is executing it. For example, some jutsus where a ninja is shooting out a stream of fire, as long as your technique is starting to execute 0.5 seconds after his, it's valid. For example, both of the ninjas can perform the handseals at the same time, but ninja one starts shooting a stream of fire BEFORE ninja two, even if its a split second before.
