[SECGolf]

How do I use the tricep to sustain lag?

The tricep or pivot causes the clubhead to change (direction and/or acceleration). You sense this change (labeled as Pressure) in PP #3.
That's why the mandate to KEEP making sure the hands are being driven (by pivot or tricep), so you can continue to sense clubhead change, and then you can direct the thrust according to your chosen plane line.

Not sustaining club head lag is not being able to sense the clubhead. So do what is necessary not to throw the club head away. Make sure the hands are being driven, shorten the stroke, clear the right hip, etc.