A squinting modifier is a modifier (usually an adverb) which could feasibly modify the words before it or the words after it. A squinting modifier is a type of misplaced modifier.
Examples of Squinting Modifiers
Here are some examples of squinting modifiers (shaded):
Cycling up hills quickly strengthens your quadriceps.
(In this example, the adverb quickly is a squinting modifier. It is unclear whether quickly pertains to Cycling up hills or strengthens.)
Taking a moment to think clearly improves your chances.
(In this example, the adverb clearly is a squinting modifier. It is unclear whether it pertains to think or improves.)
Fixing a Squinting Modifier
Like most misplaced modifiers, a squinting modifier can be fixed by changing its position in the sentence or by rewording. For example:
Cycling up hills strengthens your quadriceps quickly.
Taking a moment to think clearly about the issue improves your chances.
A Squinting Modifier is a Type of Misplaced Modifier
A squinting modifier can feasibly "look" left or right. A normal misplaced modifier (i.e., one that isn't squinting) might also feasibly modify one of two things in a sentence, but it does this by being ambiguous.
Here is an example from the film Hot Fuzz:
"He was a hero at his last police station. He once shot a robber with a Kalashnikov."
"Great, where did he get that?"
"No, the robber had the Kalashnikov."
In this example, the adverbial phrase with a Kalashnikov (a type of assault rifle) is a misplaced modifier. It cannot be squinting because there is nothing to its right. However, it is unclear whether it pertains to shot or a robber.