NewTek TalkShow VS-100 - Manual (Page 73)

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What’s “Genlocking” All About?
The term “genlock” refers to
“generator locking”.
Professional video devices often
provide a “genlock input”, which
allows an external reference signal
(often referred to as ‘house sync’)
to control its video timing.
The output of video devices
connected in this manner is
synchronized to the reference
signal, and they are referred to as
‘genlocked’.
This synchronizes TalkShow’s output to other external equipment locked to the same
reference.
Miniscule local timing differences between
video sources in a studio environment can
delay switching operations very slightly, or
even cause glitches in downstream video.
Genlocking is not a requirement, but it is very
beneficial, and you should definitely use it if
you have the capability.
Thus, serving i) TalkShow’s Genlock input and
ii) other video devices in the chain with a
single reference is the best approach.
You could think of it this way:

Genlocking your cameras has the effect of
locking their output together, ensuring
optimal synchronization for live switching.
This may result in throughput latency
benefits.

Supplying the same sync source to TalkShow’s Genlock
input ensures a match between its output and any downstream
video devices required to handle both it and other (genlocked) sources.
Locking all devices to house sync is important, but this alone does not actually ensure a
perfect downstream match. Consider an army marching along: each step the soldiers
take occurs at precisely the same moment, so we could say their timing is synchronized.
Even so – problems result if one soldier leads with the left foot while everyone else is on
the right. Or perhaps everyone is evenly spaced and perfectly aligned but for one misfit
who ‘tailgates’ the soldier ahead of him and keeps stepping on his heels.