The Gemora brings that Rav holds that the creator of a Bor Birshus Horabbim is not chayav for the chavota but for the hevel of the bor. What does 'hevel' actually mean? I have come across two possible explanations.

One is that the animal falling into the Bor head first is short of air when it hits the bottom and this causes suffocation. This explains why hevel is only a problem according to Rav when the animal falls head first. But when if falls backwards - head up - there is no issue of suffocation. However the Gemora on 53A (Rabba's answer) says if the ox fell face down and then flipped over as it was falling, Rav would agree that the baal habor would be chayav. Rashi explains that the hevel that it encountered face down at the beginning of the fall took effect and killed it. The beginning of the fall would only take a few seconds and would not be long enough to cause any suffocation.

On 54A, the Gemora asks, according to Rav, how would there be a hava amina that keilim should be chayav in Bor, what effect can the hevel on keilim? The Gemora answers we are talking about new earthenware keilim that have not been fired in a kiln that could be affected by the hevel, causing them to split. The explanation of hevel being normal air causing suffocation would not explain this case.

Another potential explanation is that the hevel is poisonous air or gas in the Bor that has contributed to the death of the animal. This therefore could affect the animal even in a few seconds (Rabba's case). We would have to say that if it falls face down it becomes affected by the gas. But if it is facing up, the air from above would 'dilute' the effect of the poisonous gas and not contribute to the daeth of the animal.

In the case of the keilim, we could say maybe the air is damp/cold and could cause the new keilim to split. But the keilim break as soon as they hit the ground. Any effect of the air would be negligible as it is only over a few seconds before impact.

In any case, the poisonous gas explanation is shver, as why should we say there is a poisonous gas at the bottom of every Bor? If you dig 100 Boros, you would not expect to find poisonous gas in the vast majority of them?

Yisroel Alter Pacanowski, Melbourne, Australia