-Turns out that Heavy D!'s power-up DM (qcfx2+K) in
extra mode (or Ult-Extra) has added attributes when performed on red health without the gauge--the "cool-down" effect of the flashing red health doesn't begin until
after the power-up DM is over. That means, you are essentially performing two DMs for the price of one: the power-up DM
and the Rush DM (qcf~hcb+P) or the Cannon DM (qcfx2+P).
-If use just the gauge for the power up DM, you cannot do another DM if you're not in red health.
-The Power-Up SDM also has the same property allowing you to do another DM, but only at the DM level.
-Basically, when in the aforementioned conditions (red health, no gauge), you should always do the power-up DM because of the free DM property.
-Note if you do the power-up DM from red health, you cannot charge your gauge manually (via ABC or getting hit) until the DM wears off.
-Hence, if you wanna do the fancy combo: power-up'ed

+P > QMAX > Cannon (S)DM...you have to power up via

~dwn+P and not the DM.
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Basics:
-d.Bx2-3, d.A > f.A >

+C
-Choose your poison: d.Bx5, d.A > Rush DM vs d.Bx3, s.A, > f.A > Rush DM does ¤almost¤ the same damage.
-Power-Up'ed ground slam

+K > s.C
-d.B, d.A >

+P > ...
-Power Up DM~(d.B), d.A >

+P >

+B > s.C (This easy combo does around 35%)
-Don't forget to add a crossup C or B before all combos.