How exactly does a partial clog cause the bowl water to drain out?

[moar]

When there's a partial clog, the bowl water sometimes drain below the normal level. What's the mechanism behind this?

 

[Terry]

It may be that the water wicks out like a wick in a kerosene lamp. The water gets pulled through. Maybe..........

I've also heard that dental floss will do the same thing. Pull water from the bowl it's it's in the trapway.

 

[Sponsor]

 

[Sylvan]

On the master plumbers exams they asked how can a trap loses it seal (8 ways) one is "capillary action " as Terry stated

 

[Reach4]

I think the partial clog keeps the siphon action much longer. By the time the siphoning down is done, the bowl refill from the fill valve has partially or totally finished. So refill water is included in the water that siphons out to the pipes.

 

[Terry]

On the master plumbers exams they asked how can a trap loses it seal (8 ways) one is "capillary action " as Terry stated

Okay, now we need the other seven. We didn't get that on our exam. I can see siphoning as one.

 

[Reach4]

Okay, now we need the other seven. We didn't get that on our exam. I can see siphoning as one.

Pets.
Cracked bowl.
Bad venting
Hurricane low pressure gusts.
Evaporation.
Washing stuff in the toilet without refilling.