Where is the signal blocked?

Crossover.

All frequencies travel down both runs of a biwire cable to the speaker. Low frequencies are not feed to the mid/tweeter due to the crossover as in single wire.

Some equipment has built-in band-pass filters in the receiver/control unit.
Usually, the speaker's crossover will reject the frequencies not needed by the given driver(s) (passive bi-wiring). 

In the case of a speaker that provides separate low frequency and mid/high frequency terminals (that are truly separate; there may be some designs in which they are not), the crossover will minimize or eliminate the low frequency content of the current that flows in the cable going to the mid/high frequency section of the speaker. It will also minimize or eliminate the mid and high frequency content of the current that flows in the cable going to the low frequency section of the speaker.

The voltage appearing at both pairs of speaker terminals will be essentially the same, however, and will be comprised of all of the frequencies that the amplifier is putting out. Although that is arguably irrelevant, as current flow in each of the cables is arguably what matters.

Regards,
-- Al

koestner,
each of the above members has given you the correct answer. The thing to remember is that in (the poor man’s) bewiring (that you are asking about) it's all about controlling the impedance between the amp & speaker. By having a biwire between the amp & speaker, you force the speaker terminals to "see" the low output impedance of the power amp as opposed to having a single wire + a jumper across the speaker terminals that would bring the impedance of the x-over circuit into the picture additionally. Many speakers benefit from "seeing" the lower impedance of the power amp & that manifests itself in better bass control most notably.
"poor man’s biwiring" because in true biwiring the x-overs are external to the speaker & they can be optimized. Often these external x-overs are active (not passive as they are inside most speaker cabinets).