Hold the bottle in one hand as normal and instead of hitting the back of it, hit the underside of your wrist instead ie hitting upwards not downwards.

The bottle and the liquid are two "things" which are loosely connected together via "fluidy" friction etc. When you hit the back of the bottle, you are effectively trying to force the bottle around the ketchup that is trying to dribble out downwards. Gravity is trying to drag the ketchup out. The impact will make the ketchup a bit more liquid. Some ketchup will flow. We know it does because it does!

When you make an impact upwards, then you are making the bottle try to leave the ketchup behind (inertia). Combine that with gravity and the impact liquidising the ketchup and more of the stuff comes out.

There are other considerations such as the shape of the bottle. If you want the fastest deposition then probably something like a conical bottle with 45deg at the apex or whatever minimises internal surface area for a given contained volume. However that will dump an entire bottle in one go and you will impale your hand if you tap the back of it. It's quite a tough problem. The squeezy jobbies seem like a bit of a cop out somehow!

Assume a spherical, infinite ketchup bottle. Be careful with your aim.

That's also Heinz's own recommendation.

* https://hpsauce.co.uk/trivia

Don't hit the bottle. Keep the lid on, invert, and shake vigorously, until flow. Then remove the lid and shake gently to apply.

Are you still able to find the glass bottles? I read a couple years ago, to my dismay, that Heinz was discontinuing them.

I've always heard this is by design.