I think those two charts are useful when used alongside each other in correlation.
It is logical that decline in BTC price leads to inflow in USDT and vice versa. So we could say (and the chart shows, that the correlation between the price BTC and market cap of USDT is inversely proportional).
For further simple explanation there are some events marked on the chart:
Red Flag - A gap in USDT inflow probably shows that institutions are selling before BTC price declines. It is actually only the third candle on USDT when the selloff of the bitcoin occurs.
Blue flag - USDT cap fails to pick up momentum and start going down significantly while BTC surges.
Orange flag - USDT market cap is flatting out, fails to go lower low and starts forming local higher lows. BTC is still pushing higher, but the trend is converting to the broad /Trading range as more and more bears are buying into the shorts and more and more bulls are taking profits.
Purple flag - BTC - huge formed and tend to break to the lower side. Lower high also formed and after.
3rd higher low formed and money starts to flow into USDT.
Green flag - top in USDT, while bottom in BTC . Reversal on both charts.
At the moment USDT market cap is in sharp decline, which might signal that BTC will be going much higher