Leapfrogging

“	It's a highly complex and dangerous undertaking, but by leapfrogging from one loop to another—a day fifty years in the past, for instance—then you'll find you have access to a whole range of loops that have ceased to exist in the last fifty years. Should you have the wherewithal to travel to them, within those you'll find still other loops, and so on exponentially.	”
–Millard Nullings, Miss Peregrine's Home For Peculiar Children by Ransom Riggs, page 337

Leapfrogging is an act where someone uses a loop to gain access to other, usually older, loops. These other loops may have collapsed by the present day but if they were still open during the other loop's date, then they will still be accessible.

Dangers

In order to visit the ancient world, one must visit dozens of loops, no fewer than 21, and track endlessly across Europe and Asia Minor, with the world growing more barbaric with each leap backward in time. It is so complicated and dangerous that it is considered impossible by temporal travel agents.

The Dark Ages is the worst time to travel through. Besides the lack of infrastructure, the people are deeply ignorant, paranoid, superstitious, and prone to extreme and sudden violence. As anyone from the future would be incredibly alien to them, their first instinct would be to kill the leapfrogger.

Miss Peregrine's Home For Peculiar Children

After Miss Peregrine is kidnapped, her wards make a plan to search for her. They realize that the wights who took her may have leapfrogged so on top of figuring out where the ymbryne was, they would also have to find out when, especially because she could not stay outside a loop for long or she would rapidly age. The children then use Miss Peregrine's loop to attempt to find another loop or place that would lead them to Miss Peregrine.

The Desolations of Devil's Acre

To get to Miss Tern's loop, leapfrogging had to be used to arrive safely and within a a day or two. The peculiars going to find Miss Tern's loop end up first going through Miss Hawksbill's loop as Miss Tern's had collapsed a hundred years from the present. Miss Hawksbill's loop was the only loop that could be found with a short travel time and relative safety.