Maxbert
In (I hope) laymens terms. a PTU uses hydraulic pressure from 1 system to drive a transfer shaft. The non pressurised system has an impellor in it which thereby pressurises the second system with no fluid transfer between the two systems.
An analogy would be hydro electric power - water under gravity (in this case hydraulic fluid at 3000psi) drives a shaft which turns a turbine (in this case an impellor sitting in the hydraulic line) generating electricity (in this case hydraulic pressure). There is no mixing of input and output sources.
HTH