How do I use the trim function correctly on Shimano shifters I have Shimano SL-4700 Tiagra (flat handle) Double shifters. However the front shifter has 4 positions in this order:

High 

High trimmed

Low trimmed

Low

As can be seen in the image below. 

However, I can't make sense of how this is meant to work because there is no way to move between the untrimmed settings. For example, if I want to jump to the small cog from the big cog (from H to L), I would expect to simply pull the top lever with my finger, but in fact it just goes down to the Low Trimmed so I have to do a second pull.

Furthermore, one of the trims has little effect anyway, and I find trim useless anyway as I never cross chain. I'm wondering if Shimano intends you to only use 3 of the settings (i.e. (1) H/HT + L, or (2) H + LT/L) by judicious setting of the limit screws, but I can't find any info about this. My old 8 speed Claris had only one trim, which I could bypass when changing. Not being able to go from high to low with one input seems weird, and I can't imagine Shimano would design the system like this. 

Finally, when adjusting the cable tension and limit screws is there any special technique to deal with the trim.
H: To be used with the smaller cogs of the cassette.

HT: To be used with the larger cogs of the cassette. You might hear a rubbing noise  of the chain on the inner blade of the front derailleur if there's a large cog in use at the rear and the front is still on H.

L: You'll use it when the chain is on one of the larger rear cogs.

LT: For when the chain begins rubbing on the FD when you shift on one of the smaller rear cogs.

On a safe flat road put the chain on the smallest cog, the biggest ring and start shifting the rear derailleur towards larger cogs. Look down on the FD and not the behaviour of the chain between the two blades of the FD. At one point, you'll notice the chain touching the FD. That's the point where you trim the FD.

Note: when you shift at the front, the derailleur will go to L or H. Depending on the situation at the rear, you'll have to trim at the front right after the chain has fallen on the small front ring or climbed on the big ring.