In the above diagram, all four variants are synched. The Shifters are physically at Point 1 - at Nexus - but have synched variant 2 to variant 3. What actually happens is that Nexus is synched to variant 3 at this point (because that is where Shifters actually are), but this is equivalent to variants 2 & 3 being synched, since the synching runs through variant 2.
If the Shifters are hosted into bodies in variant 4 and enough time passes for variant 3 to collapse, they will be involuntarily punched out, because they will no longer be synched to the timeline they are hosted in.
Two variants can only be synched at one point. If someone has already made a synch hole from your current variant to a target variant, you cannot make another synch hole and enter there. This means if a group of Shifters operating from Nexus make a synch hole in a variant, no other Shifters from Nexus may enter at a different point in time in that variant until the synching is released. They could, however, synch with another timeline, splice themselves in, drop their synching with Nexus and then make a new synch hole to the target variant.
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