Unlike lithium aluminum hydride, which is reducing everything in sight, LiAlH[OC(CH3)3]3 is a lot more controlled. First of all, it only has one hydride to give, unlike LiAlH4, so it’s a lot easier to control the reaction using stoichiometry. Secondly, those big bulky tert-butoxy groups (that’s -OC(CH3)3) help to modulate (i.e. slow down) the reactivity of the reagent. They’re kind of like a fat suit around aluminum that ensure that the hydride can’t fit into tight spaces.
This serves as a way to indirectly reduce carboxylic acids to aldehydes: you can convert the carboxylic acid to an acid chloride using something like SOCl2 or PCl3, and then reduce the acid chloride to the aldehyde with LiAlH[OC(CH3)3]3 .
the hydride from Al–H adds to the carbonyl carbon of the acid chloride, breaking the C–O π bond and forming a tetrahedral intermediate. If you’ve covered carbonyl chemistry at all, you should recognize this step as The Most Important Mechanistic Step in Carbonyl Chemistry – the 1,2-addition. Then, we’ve got this negatively charged oxygen which can then come down and re-form the C-O π bond, expelling the chloride ion (Cl-) in the process. This is the 2nd Most Important Mechanistic Step in Carbonyl Chemistry, called the 1,2-elimination.