  A 20 second cadence TESS monitoring campaign of 226 low-mass flare stars
during Cycle 3 recorded 3792 stellar flares of >10^32 erg. We explore the
time-resolved emission and substructure in 440 of the largest flares observed
at high S/N, 97% of which released energies of >10^33 erg. We discover
degeneracy present at 2 minute cadence between sharply-peaked and weakly-peaked
flares is common, although 20 second cadence breaks these degeneracies. We
better resolve the rise phases and find 46% of large flares exhibit
substructure during the rise phase. We observe 49 candidate quasi-periodic
pulsations (QPP) and confirm 17 at 3+ sigma. Most of our QPPs have periods less
than 10 minutes, suggesting short period optical QPPs are common. We find QPPs
in both the rise and decay phases of flares, including a rise-phase QPP in a
large flare from Proxima Cen. We confirm the Davenport et al. (2014) template
provides a good fit to most classical flares observed at high cadence, although
9% favor Gaussian peaks instead. We characterize the properties of complex
flares, finding 17% of complex flares exhibit "peak-bump" morphologies composed
of a large, highly impulsive peak followed by a second more gradual Gaussian
peak. We also estimate the UVC surface fluences of temperate planets at flare
peak and find 1/3 of 10^34 erg flares reach the D90 dose of D. Radiodurans in
just 20 seconds in the absence of an atmosphere.
