The Caps are on the hook for $4.5M per each remaining year of the $11M/yr contract. Post rollback, that's $3.87M the Caps are paying, and $5.59M the Rangers are paying each year left on the deal.
For the purposes of the salary cap, the league determined that the contract is split in half: for the nearly $9.5M Jagr receives each year, post-rollback, about $4.75M is charged against each team's salary cap figure.
This is presumably fine for the Capitals since they have no intention of maxing out their cap. The Rangers, on the other hand, probably will be very close to the cap. Works for everyone. Since Jagr's cap percentage is halved between the Caps and Rangers, it is not an issue in terms of percentage of a team's total budget. He is only taking up $4.75M of the Rangers' cap space, even though his actual salary is almost $9.5M/yr.
That should cover it.