Not sure if both diesel and petrol have the "pwr" button but it feels on my '96 petrol like it does a bit more than just hanging onto the gears longer. Genuinely seems to make more power in the same gear (I'm wondering if it advances spark a bit?). Makes a fair bit of difference when towing. Also uses a LOT more fuel, maybe it also runs a tad richer in PWR mode? Mine is LPG converted and the PWR button makes the same difference on either petrol or LPG
The auto has it regardless. Makes no difference to fueling etc apart from uses more.
The petrol makes peak power higher up the revs (around 3200rpm as standard) so it feels like more power as your using what it's got.
Try driving it like a manual up a steep hill, 1st (L) then 2nd before go into drive but using all the revs, you'll feel it as power starts to drop off towards top end of the revs (4.5 to 5k rpm you'll feel it flatten off)
It's more noticeable on bigger tyres.
It also doesn't lock the torque converter for a while so uses more fuel.
Mine locks about 45 mph not in PWR mode, in PWR it can be 70mph.
I quite often change between 2nd and D on winding, hilly roads as keeps the revs up, few roads in the Yorkshire Dales its L and and 2nd due to steep hills, just have to not be scared to rev it.
As said with CDL switch and disconnect the plug underneath makes a nice drive in low range.
When I'm laning even on steady gentle terrain I use low range, 2nd start pressed and in D.
Just smoothly potters along 15 to 20mph, below 1500rpm. Manually select 2nd to slow down and L to go down any steeper hills which means less braking needed. Manually engage 2nd for some uphill rocky bits and if more technical just put in L.
CDL switch if start having traction issues or likely to, rear and front diff locks means it's got interesting but tbh rare it needs more then the rear.
Passengers to start with seem surprised how much I use the gearbox and even low range (it's highly capable in high range)
When leave it in D high or low, on or off road they see why I do it