One of the examples of the word Pucker (noun) in https://www.merriam-webster.com/dictionary/pucker is "Northern California’s Rubicon Trail—one of the most iconic and difficult off-road routes in the world—makes a typical driver totally pucker." I don't understand (1) the real meaning of "pucker" in this sense and (2) why "totally" (assumed as an adverb) is placed in front of a noun. Could you shed some light please? Thank you so much.
Your example is a colourful phrase from the lead paragraph of a current (2018) US magazine about off-road driving.
To pucker is a verb meaning to wrinkle. Here it might politely mean purse his lips (as if eating something sour) but given its context, is much more likely to mean what commenter (thanks!) described as "anal contraction caused by fear", per Urban Dictionary.
OED: 1. intransitive. To gather or contract into creases, small folds, or bulges; to become drawn together into wrinkles or corrugations.
The adverb totally here just is emphasis.