After playing online for awhile, I sometimes noticed how odd sometimes I would punish someone who was getting up or just missed an attack while their Synergy Burst Gauge wore off.
When you're Gauge empties you do a tiny animation to show it, but apparently for a few frames during this animation, you cannot attack, counter attack, or jump.

But Shield seems to work from frame 1, not 100% sure on all the frame data on this, but I realized if shielding was your only option for these few frames, then it could be a free grab on your opponents Synergy Exhaust Frames (SEF).

With good timing, you can grab your opponent from these frames for a free punish, or get a free combo from it if they don't hold shield upon an empty gauge after getting up or attacking.
This is just from a bit of testing. I'm not sure this is 100% guaranteed but it sure feels like it.

The last clip of the video is the one that made me want to look into this.