Using Thumb Pressure to Open Up a Crack in Glass

Question:

Can I use thumb pressure on the glass from inside the vehicle to help open up a crack?

Answer:

Yes, but you cannot usually flex the glass and open up a crack from the inside and work on the outside at the same time, so a crack expander may be more effective. Too much flexing will cause the crack to pull in air, so it is best not to flex at all unless the crack will not fill without doing so. The crack expander may also be used to open stubborn star breaks.