Reverse dieting.

Reverse dieting is carefully increasing your calories after a period of drastic deficit with the goal of maintaining weight. Bulking on the other hand is eating a calorie surplus with the goal of gaining weight. Reverse dieting is a means to bring back to normal, a persons BMR that has been damaged due to drastic dieting. It is not as effective on a person who has not had damage to their BMR from drastic dieting and simply wants to try and increase their normal BMR. Utilized properly it is an appropriate intermediary step into a muscle building bulking phase following a fat shredding cutting period. 

In short in Norwegian: 
Når du vil beholde vekta:

Reverse dieting er å øke kalori-inntaket med med 50 - 100 kalorier fra karbohydrater eller fett  daglig  i syv dager , deretter innta 50-100 oppå der daglig i neste uke. Altså hvis du normalt inntar 1500 kalorier skal du innta 1550 i syv dager, 1600 de neste syv dagene osv. Vei deg ukentlig. Flere uker kan gå uten at du går opp i vekt. Når du så ser at du har gått opp skal du slutte å innta de 50-100 kalorier ekstra, men holde deg på det antall kalorier du har kommet opp til. Du skal på denne sakte måten unngå vektøkning. Hvis du i utgangspunktet trenger å redusere vekten kan du gjøre det når du har kommet frem til max kalori-inntak, og redusere inntaket med 15 % av dette max- kalori-inntaket. Men vent til kroppen har blitt vant til det økte inntaket en stund.

The best thing to do is to slowly increase your DAILY calories by 50-100 each week. With this small amount you should not see much change on the scale. If you do, you can remain at the same number of calories for another week and try increasing the following week. If you jump from 1200 to 2000 calories overnight, there will likely be fat storage, but increasing your calories by 50-100 on a weekly basis will give you body a chance to adapt. Whatever you're eating now start increasing by 50-100 calories per week. Continue doing this until you hit maintenance calories. Then, cut your intake by 15%. So if you're eating 1500 calories per day this week, next week it would be 1550. Carbohydrates are most effective at raising leptin levels as compared to fat and protein. 

After reverse dieting for a period of time you will come to a point where you have maxed out on the level at which you can increase your maintenance calories (without gaining weight).Once you are there and the world is at peace you have two choices to make: continue increasing your calorie consumption and begin bulking (aka adding muscle) or start back towards the way you came and drop body fat through a calorie deficient diet.  With either option, it is wise to stay at your new maintenance calorie level for a period of time to give you and your body some homeostasis and a fresh point at which to start your new transition.

For underweight people:

In order to gain weight and build muscle, you simply need to eat more. Try adding something additional each day for a week, approximately 50-100 additional calories. If you do not start to see any weight gain, add something else to your daily intake to increase another 50-100 calories. Keep increasing your intake slowly until you start to see your weight increase then hold at those calories until you stop gaining and slowly add more again. If you are recovering from ED, continue reverse dieting until you are at a healthy weight and maybe even continue into a bulking period and gain a little fat. If you are reverse dieting correctly you should only be putting on minimal fat, so don’t be overly alarmed. 

http://averagefitnessfreak.com/reverse-dieting-eat-more-and-maintain-weight/

How many calories? :Take your current body weight in pounds and multiply it by 14 and 17. Somewhere in between those 2 amounts will usually be your daily calorie maintenance level. A 180lb person would do 180 x 14 and 180 x 17 and get an estimated daily calorie maintenance level of somewhere between 2520-3060 calories. People who are female, older, less active or feel they have a “slow metabolism” should probably stick more towards the lower end of their estimate. People who are male, younger, more active, or feel they have a “fast metabolism” should probably stick more towards the higher end of their estimate.