When they say a chemical flush they only mean they add something like a BG Flush to your AT, run it for 20-30 minutes and then hook up the flush machine. A "power" flush is not really anything to do with power. It just adds new fluid to the sump under a slight pressure at about the same rate the AT pump pushes fluid from the sump through the transmission and out into the flush machine.
If you take really good care of your AT then a flush replaces 100% or maybe 95% of the fluid and is better than a pan drop.
The better places can hook up their flush machine, drain the pan, then they remove it, clean it and replace the filter, then the flush machine adds more fluid and finishes with the flush.
The real key here is knowing the condition of the transmission, filter and pan.
Many people are misinformed that a flush pushes new ATF under pressure though the transmission loosening built up crud. A pan drop & refill or a flush will each add enough new ATF to the system to start to clean things up. One could argue that 100% new fluid loosens crud faster than 50% new fluid, but thats a bad bet to take. If you do not know the condition of the pan, then it needs to be removed, cleaned, filter replaced amd then your choice of a flush or refill.
Once you have things under control, then a program of fluid freshening might be in order especially if you have gone to synthetic ATF. That is drain and refill the pan every 20K miles (synthetic) or so. You remove used fluid with wear metals and replace with fresh.
If you have added a Magnefine filter then you can probably bump up the miles for a freshening. Given the cost of synthetic ATF, a Magnefine would always seem to be a good value.
Amsoil ATF is rated for severe at 50K and normal at 100K. But you need a filter to run those numbers.
