"Defense Matrix, activated! "
As stated, the trick is to do it as Torb is shooting the lava, intercepting it. Nothing can be done once it touches the ground/payload
Actually, once upon a time that was his Ult until the October 9th, 2018 rework he received in OW1. It basically had him enter what is now Overload, and his sentry upgraded to a Level 3 for the duration of the ult, gaining increased damage and health.