No PSK in Overtime - Explanation??

[GPC2]

In helping another official study, we came across a question that talked about PSK occurring in overtime. While I know that PSK is not valid in overtime, in my attempts to explain why, I realized that I couldn't. Can anyone give me a good explanation of why PSK is not valid in overtime?

Example: In overtime, 4th & Goal from R's 8-yard-line. K's field goal attempt sails into the end zone, but wide-right of the goal posts. R blocks below the waist at it's own 3-yard-line, prior to the ball becoming dead.

Is the enforcement from the previous spot, or succeeding spot (10-yard-line)?