The key technological innovation enabling this new business paradigm? Electronic signatures a legally binding, secure version of the handwritten signatures you use to sign paper documents today.

这个问句和后面的陈述句都是省略句，都只是一个名词词组。

The key technological innovation enabling this new business paradigm? 这是一个设问句，作者自问自答。省略了系词和作表语的what.

(What is) the key technological innovation enabling this new business paradigm?

enabling并不是谓语（非限定动词怎么能作谓语？），而是innovation的定语。

Electronic signatures a legally binding, secure version of the handwritten signatures you use to sign paper documents today.

这个句子没有谓语动词，是设问句的答案，省略了主语和系词be.

(It is) electronic signatures, a legally binding, secure version of the handwritten signatures you use to sign paper documents today.

其中，a legally binding, secure version of the handwritten signatures you use to sign paper documents today.是一个以version为中心词的名词词组，作electronic signatures 的同位语。