Thumbs are like gator mouths. Seriously, pinch your other hand with just your thumb then try extending that same thumb outward while your other hand holds it down.
I always hated when that happens. I never understood how the opponent's thumb were so strong and it looked like they were barely trying.
the trick's pretty easy, IMO. try reaching your opponent's thumb's base, with his entire thumb under yours is better. that way, your opponent won't be able to lift his thumb at all
