An open sample switch isolates the storage node, so this ideal ledger keeps the previously captured voltage even after the input moves. The page does not claim a real decay curve; hardware hold accuracy depends on leakage, dielectric absorption, buffer loading, switch off resistance, elapsed time, and measurement tolerance.
Once the sample switch opens, the held value is the model output. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The open switch isolates the held node
The input has moved to 1 volt, but the held output remains 4 volts.
Vout=Vheld=4V
No decay curve is claimed here
The exact model says hold equals 4 volts. Leakage and acquisition time are deferred.