On 8b we assume if the heker is for people in the mavui then it is not for one standing underneath but rather they would see it from the inside and one should not use the area directly under the beam. (Which makes sense as it is a 3 inch beam up to 40 feet in the air and people do not walk around with their neck straight up). One 4b bottom we seem to assume that according to R yosef (see bottom rashi) the function of heker is met with 20 amos max in the area directly under the beam.
What is pashut pshat in how the heker works? Tha k you
Moshe rubin, Brooklyn, ny
There is a dispute on 8b whether or not one may use the area under the beam. Rav Yosef on 4b agrees with the opinion that one may use that area. Since that area is less than 20 amot high this means that it is low enough to be able to see the korah so there is a heker. The opinion you cited on 8b disagrees with Rav Yosef.
KOL TUV
Dovid Bloom
Follow-up reply about how heker works:
1) Since a mavui is a place that is close to Reshut Harabim, and is also a place where there are a lot of bypassers, it would be very easy for people to get the wrong idea that since one may carry inside the mavui (because Shitufei Mevo'ot was done there) so too one may carry in Reshut Harabim. To prevent this mistake, Chazal required us to put a korah near the entrance to the Mavui to distinguish between the Mavui and between Reshut Harabim and people will understand that it is only permissible to carry in the Mavui because it is significantly different from Reshut Harabim.
2) There is a dispute on 8b whether the heker is necessary fir the people inside the mavui or for the people outside the Mavui.
Dovid Bloom