Hi! Thank you so much for playing the game even though it is very incomplete and is just a sandbox! (I wouldn't even consider it a game in its current state lol)
Throw mode is something that only exists in the sandbox. Really all of the UI is sandbox mode and will be removed in the final version.
If you do want to give throw mode a try, you can click on the normal mode button and then it will switch to throw mode. You then left click with dices inside the blue box, hold and drag to where you want to throw and then release. It kinda works like the split dice but you use left click instead of right click. I have also updated the instruction a bit to make it clearer.