I have seen previously that people have tied a rope to the trailer and to the tow bar and used a knot that acts like a pulley, where you can unhich the trailer feed the trailer further down the slipway.
Hi Leigh
Welcome to the forum.
I'm guessing you have a bunk trailer and therefore have to completely submerge the trailer to float the boat off. The pulley knot you've mentioned sounds like a simple truckers knot, which is basically a loop in a rope and the loose end around the tow hitch and through the loop in order to aid the moment of inertia.
I'm more simplistic with my approach though and have merely driven the trailer onto the ramp enough that it will roll backwards if disconnected. Chock the wheels or apply the brake, disconnect it, drive the car further up the ramp and attach a rope to the hitch and tow ball via bowlines either end. Then merely use the brakes to allow the car and trailer to roll backwards into the water. Recovery is the reverse.
Bowline knots are probably the most used knots because they can be broken when not under pressure but will take enormous weight almost to the breaking strain of a rope. Jobs they are not suited for are, for example anchors where its possible that the knot can come loose when not under pressure, under pressure it will never come loose.
There are loads of vids and websites describing how to tie them, but even after 50 yrs using them, trust me sometimes I have complete brain fade and have to think about it .
This is one of the best examples...………..
https://www.netknots.com/rope_knots/bowline