relinquishing timeslices in a transaction

When the Scheme48 virtual machine delivers an interrupt, it saves the
current proposal, to be restored when control returns from the
interrupt handler.  This is how per-thread proposals are preserved
when switching threads in the timer interrupt handler, but how are
they preserved when switching threads using RELINQUISH-TIMESLICE?  I
don't see any evidence that it saves the current proposal anywhere.