Does your bike has a front derailleur and do you want to operate both the rear- as the front derailleur with the same hand on a dropbar bike, than you can use a mechanical time trial shifter.
A lot of bikes nowadays come with an electronical groupset. But when you buy / own a bike with a mechanical groupset, than you can use this lightweight and simple solution. Be sure you mount a front- or rear derailleur specific time trial shifter, because of the indexing!
If you are right-handed, you are fortunate that you do not have to operate the rear derailleur with the “extra time trial shifter” but can use the standard shifter in the brake lever (STI) for this. You shift most of your gears with the rear derailleur, as it has the most “cogs” (sprockets on the cassette), and at the front you usually only have 1, 2 or 3 sprockets (most racing bikes have 2 front sprockets). You therefore only switch from the front sprocket when there is a strong headwind or when going uphill. Furthermore, you will usually use the rear derailleur.
Because the left standard shifter in the brake lever (STI) has an “index” for the front derailleur, so usually only has two positions, you cannot operate the rear derailleur with it.
If you are only left-handed, electronic shifting is therefore all the more worth the investment.