The knee frequency is an indication of how fast the signal
change( a square wave, I mean) from 0 to 1. The ratio knee/3-dB frequencies
is an indication how much time is left of margin. In other words,
if you have a digital signal which as 400 ps period but you have a rise time of
100 ps then you have 300 ps of margin. If your setup or hold time
is greater than that then you need to decrease the clock rate. If you add
some others restrictions as phase noise/jitter for example then you
margin is further reduced. In high speed digital design you want a clock as fast
as you can but there is a limit, the signal must be stable in other to be error free.
Added after 13 minutes:
I forgot.... even if you have a driver and a receiver that they have
enough margin to talk to each other, they need to be connected by
a physical medium, the channel(coax cable, RS232, fiber,etc). If this
channel has restriction regarding bandwidth , in other words, the
3-dB bandwidth of the channel is lower than the knee frequency of the
signal, then the rise time of the signal will be increased and lowering your
margin. If you add ringing(mismatching) and settling time to it you will
have further reduction in margin.