at the moment it's just a vote and on 08/01/2017 it will become a separated blockchain.
Technically, it will not become a "separate" chain until the 1st segwit signaling block is found on/after 08/01/2017 (it becomes a "stopchain" instead of a blockchain). If no such segwit signaling block is found on/after 08/01/2017, it just stops at the point where the existing chain was and "orphans" all of the new non-segwit signaling blocks in it's own mind.
Well, as I understood, some minority fraction of miners was supposed to fork away on a pure-segwit chain on 15/11. In fact, the 8/1/2017, nothing particular will happen if I understood well. I think one explained to me that the UASF nodes will not STOP on a non-segwit block, but will simply "not count" it in their percentage of segwit signalling, tricking themselves in thinking there's 100% segwit support, even though they accept non-segwit blocks too in the chain.
What was less clear to me was whether these nodes will also not consider *transactions* in these non-segwit blocks, or whether they will pretend that those transactions never occurred.
At the "switch-over point" (August 1st 2017 (epoch time 1501545600)), "Blocks that do not signal as required will be
rejected". So if the next 10 blocks do not signal "according to the existing segwit deployment", the UASF chain just "stops" until the 1st such signaling block.
Well, there remains something unclear about this, and when I ask, I get alternating answers
The explanations I heard before were these:
explanation 1:
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On 1. August 2017, UASF only CONSIDER segwit-signaling forks on the main chain (but for one or other reason, they don't mind them being successors to non-segwit blocks). In other words, there is still only ONE CHAIN, but the UASF node "picks out" only the segwit signalling blocks.
This gives the *impression* to the node, that all blocks are segwit-signalling, and hence ACTIVATES segwit on 11/15/2017. From that point on, it wants a REAL SEGWIT-ONLY chain, and maybe some miners will fork off to make one, different from the main chain. So the real forking only happens in November.
explanation 2:
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On 1. August 2017, UASF activates segwit, and doesn't accept non-segwit blocks. The only chain it can receive at that point, is a segwit-only chain, so maybe some miners will make one forking off on 1. August 2017. (in other words, what is happening on 11/15/2017 in explanation 1, is already happening on 8/1/2017 in explanation 2.
I don't know which one corresponds to what is BIP148.