It is likely that they're talking about the force necessary to secure the
knot. Too much force and the suture breaks at the knot. Too little and the
knot doesn't hold. It's actually a complex issue. If you're suturing against
opposing force, the tissue trying to spring back apart will want to loosen
the the knot of the first throw. The second throw then locks the knot, but
if the first throw didn't provide enough apposing force the knot will now be
locked and too loose. Two common solutions are to make the first throw a
"surgeon's knot", which is a double throw, providing more friction to keep
that knot from loosening before the second (locking) throw can be put on.
The other is to make the second throw in the same direction as the first, a
granny instead of a square knot, in which case applying the correct amount
of "knot run-down force" will tighten the knot and not allow it to slip
before you can get the third and fourth square throws on.

I've explained it badly, I fear. It's one of those things a surgeon know how
to do through having done it a million times, but can't articulate exactly
how it's done.

HMc