Legit: US$ are sent to tether, tethers are issued in exchange 1:1 and then used to buy bitcoin. This will tend to raise the price of bitcoin but only as long as people are sending real US$
Illegit: tether insiders just issue tethers without US$ backing, use them to buy bitcoin and probably try to sell the bitcoins at a profit becoming million/billionaires in the process. Sky's the limit really - you can issue crypto tokens in any amount though it could all collapse if people want to swap their tethers back to US$ and there aren't enough there.
>They then inject this into BTC, ETH, LTC, (and others) to cause prices to pump.
Means use the tether to buy BTC etc in exchange for tethers which will tend to raise prices if there are more buyers than sellers and people seeing the price rising will cause them to buy more.