A slap has less force than a smack. A slap on the wrist is less painful than a smack on the bottom.

还希望选择我的解释作为最佳答案吧，因为英语母语者的结论正确，但由于不会汉语细节无法说全，有时难以理解。 I humbly hope to choose my explanation as the best answer. Though English speakers are correct by conclusion, but due to not speaking detailed Chinese ,their explanations can not be complete to be full of your query and therefore sometimes difficult to understand.

1. Todd smacked down his hand on the table, it is a right sentence,but, 正常顺序写成: Todd smacked his hand down on/onto the table.意思是: 托德(用力)啪的一声拍了一下桌子。 这句话主要目的是: 想制造这种噪音(或者引起对方注意)。但不一定是托德生气而做这个动作。 (english version ): Written in the normal order: Todd smacked his hand down on/on to the table, which means: Todd smacked (with big force) the table (hard). The main purpose of this sentence is to make this noise (or to attract the other party's attention). But it doesn't have to be because Todd is angry. 2. Todd slapped down his hand on the table. " 意思同上，也是啪的一声发出噪音，但很可能是托德生气时做这个动作，是很快的且不由自主的动作。 (english version ) It's the same as above, it's also a snap noise, but it's probably when Todd is angry, it's a very quick and involuntary movement. Hope this helps.