My feeling is that pushing (bearing down) places a strain - possibly excessive/damaging - on the plumbing.
To be avoided.
I get the same thing if I've had to hold it for a while. What works for me is letting go more. How that is achieved is to start pissing, and if unhappy with the flow, gradually shut it off, then hold for at least 30 seconds (or so) then gradually let it go again.
I find that doing this allows a more conscious awareness of what muscles are being relaxed, and to then deliberately relax them further. Which results in a good stream. (Or sometimes not.But usually, when the exercise is timed right.)
Incidentally I find the flow at its worst after quite a consumption of caffeine (tea) and at it's best after beer.